METHOD FOR MAKING A CaTiO COMPOSITE THIN FILM ELECTRODE FOR WATER SPLLITTING

ABSTRACT

A CaTiO3—TiO2 composite electrode and method of making is described. The composite electrode comprises a substrate with an average 2-12 μm thick layer of CaTiO3—TiO2 composite particles having average diameters of 0.2-2.2 μm. The method of making the composite electrode involves contacting the substrate with an aerosol comprising a solvent, a calcium complex, and a titanium complex. The CaTiO3—TiO2 composite electrode is capable of being used in a photoelectrochemical cell for water splitting.

STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTORS

Aspects of this technology are described in an article “Fabrication ofphotoactive CaTiO₃—TiO₂ composite thin film electrodes via facile singlestep aerosol assisted chemical vapor deposition route” by Ehsan, M. A.,Naeem, R., McKee, V., Rehman, A., Hakeem, A. S., and Mazhar, M. J MaterSci: Mater Electron (2019) 30: 1411, doi: 10.1007/s10854-018-0411-4,which is incorporated herein by reference in its entirety.

STATEMENT OF ACKNOWLEDGEMENT

This project was prepared with financial support from the High-ImpactResearch scheme grant number: UM.C/625/l/HIR/242; FRGS grant number:FP039-2016; IPPP grant number: PG053-2016A; and HIR-MOHE grant number:UM.S/P/628/3SC21. AR acknowledges the start-up grant number: SR151005from King Fahd University of Petroleum & Minerals (KFUPM).

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a method of making a composite thinfilm electrode that is capable of photocatalytic water splitting.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Photoelectrochemical (PEC) splitting of water for generating hydrogen isbelieved to be a safe and auspicious route for solar energy conversion.This technology relies on the sun, a long-lasting source of energy,while using ubiquitous water as a renewable energy resource. For theprogression of PEC technology, efforts have been made to develop arobust and stable photocatalyst that can perform the water decompositionreaction at milder conditions to produce hydrogen, thus meetingindustrial demands. See S. Chen, S. S. Thind, A. Chen, Electrochem.Commun. 63, 10-17 (2016); T. Hisatomi, J. Kubota, K. Domen, Chem. Soc.Rev. 43, 7520-7535 (2014); and X. Zou, Y. Zhang, Chem. Soc. Rev. 44,5148-5180 (2015), each incorporated herein by reference in theirentirety. In this regard, titanium dioxide (TiO₂) and various otherTi-based oxide semiconductors are extensively researched owing to theirsignificant photocatalytic efficiency while being non-toxic, chemicallystable, and low cost. See X. Chen, L. Liu and F. Huang, Chem. Soc. Rev.44, 1861-1885 (2015); Y. Ma, X. Wang, Y. Jia, X. Chen, H. Han, C. Li,Chem. Rev. 114, 9987-10043 (2014); and E. Kalamaras, V. Dracopoulos, L.Sygellou, P. Lianos, Chem Eng J. 295, 288-294 (2016), each incorporatedherein by reference in their entirety. Ti-based composites of alkalineearth metal titanates (e.g., CaTiO₃, SrTiO₃, and BaTiO₃) are consideredbetter choices among them for splitting of water. The main reason ofsuch popularity is their unique perovskite structure. See W. Wang, M. O.Tadé, Z. Shao, Chem. Soc. Rev. 44, 5371-5408 (2015); and S. S. Arbuj, R.R. Hawaldar, S. Varma, S. B. Waghmode, B. N. Wani, Sci. Adv. Mater. 4,568-572 (2012), each incorporated herein by reference in their entirety.In such perovskite structures, the lowest part of the conduction band ismainly comprised of empty d orbitals of the transition-metal (e.g.,Ti⁴⁺) having a potential slightly negative than 0 V, which makes themcapable of oxidizing water. See W. Wang, M. O. Tadé, Z. Shao, Chem. Soc.Rev. 44, 5371-5408 (2015), incorporated herein by reference in itsentirety. However, a significant drawback of these perovskitephotocatalyts is their limited light absorption capacity, which islinked to their poor charge separation efficiency. This is the result oftheir large band gap and high trapping density. See N. P. Dasgupta, J.Sun, C. Liu, S. Brittman, S. C. Andrews, J. Lim, H. Gao, R. Yan, P.Yang, Adv. Mater. 26, 2137-2184 (2014); O. A. Jaramillo-Quintero, M. S.de la Fuente, R. S. Sanchez, I. B. Recalde, E. J. Juarez-Perez, M. E.Rincón, I. Mora-Seró, Nanoscale 8, 6271-6277 (2016); and E. Grabowska,Appl. Catal. B. 186, 97-126 (2016), each incorporated herein byreference in their entirety. Two predominant strategies have beenproposed in order to tackle these problems. See Dasgupta et al. (2014);and T. W. Kim, K.-S. Choi, Science 343, 990-994 (2014), eachincorporated herein by reference in their entirety.

Firstly, the methods of bandgap engineering have been employed to have aband edge position alignment by the development of heterojunctions or bydoping these materials with metal ions so as to configure thesematerials for spontaneous water splitting. See H. Zhang, G. Chen, Y. Li,Y. Teng, Int. J. Hydrogen Energy 35, 2713-2716 (2010); and A. Mumtaz, N.M. Mohamed, M. Mazhar, M. A. Ehsan, M. S. Mohamed Saheed, ACS Appl.Mater. Interfaces 2016, 8, 9037-9049, each incorporated herein byreference in their entirety. The perovskite-based photoanodes modifiedby this strategy (e.g., SrTiO₃/Cu₂O, BaTiO₃/TiO₂, and CaTiO₃/Pt) haveshown enhanced photocatalytic activities, thereby utilizing the solarspectrum more successfully. See D. Sharma, S. Upadhyay, V. R. Satsangi,R. Shrivastav, U. V. Waghmare and S. Dass, J. Phys. Chem. C, 118,25320-25329 (2014); W. Yang, Y. Yu, M. B. Starr, X. Yin, Z. Li, A. Kvit,S. Wang, P. Zhao, X. Wang, Nano letters 15, 7574-7580 (2015); and K.Shimura and H. Yoshida, Energy Environ. Sci. 3, 615-617 (2010), eachincorporated herein by reference in their entirety. Secondly, syntheticstrategies have been devised to enhance the separation of electron-holepairs in photoanodes, both by crystal size reduction to the scale of thehole diffusion lengths and by increasing the carrier conductivitythrough morphological and crystallographic control. However, bothapproaches are confined by the limits of the fabrication method.

In this regard, aerosol assisted chemical vapor deposition (AACVD) hasshown great promise in engineering the photoelectrodes with novelmorphologies, high crystallinity, and controlled thickness. See A. A.Tahir, H. A. Burch, K. U. Wijayantha, B. G. Pollet, Int. J. HydrogenEnergy 38, 4315-4323 (2013); A. A. Tahir, M. Mat-Teridi, K. Wijayantha,Phys. Status Solidi Rapid Res. Lett. 8, 976-981 (2014); and M.Mat-Teridi, A. A. Tahir, S. Senthilarasu, K. Wijayantha, M. Y. Sulaiman,N. Ahmad-Ludin, M. A. Ibrahim, K. Sopian, Phys. Status Solidi Rapid Res.Lett. 8, 982-986 (2014), each incorporated herein by reference in theirentirety. AACVD is highly effective in constructing bi/multicomponentphotoelectrodes with precise elemental stoichiometry, and it ensures thehomogenous coupling of the bi or tri phases in a single step, leading tothe reduction of the band gap of the material. See A. Mumtaz et al.(2016); M. A. Ehsan, H. Khaledi, A. Pandikumar, N. M. Huang, Z. Arifin,M. Mazhar J. Solid State Chem. 230, 155-162 (2015); and M. A. Mansoor,M. A. Ehsan, V. McKee, N.-M. Huang, M. Ebadi, Z. Arifin, W. J. Basirun,M. Mazhar J. Mater. Chem. A, 1, 5284-5292 (2013), each incorporatedherein by reference in their entirety. The homogeneity and the size ofthe aerosol droplets can be tuned to have a control on the morphology ofthe resulting films. This is done by controlling the viscosity of theprecursor solutions and the frequency of the aerosol generator duringthe AACVD process. See A. A. Tahir, K. U. Wijayantha, S.Saremi-Yarahmadi, M. Mazhar, V. McKee, Chem. Mater. 21, 3763-3772(2009), incorporated herein by reference in its entirety. Duringdeposition via AACVD, the particle growth and sintering processessimultaneously occur on the substrate surface to develop wellinterconnected morphological features and to produce adhesive filmelectrodes. This particle-particle or particle-conducting layerconnection improves the electrical contacts and hence, the conductivityof the nanoparticle based thin films. The result is the enhanced chargetransport properties of the resulting electrodes, thus exhibiting abetter photocatalytic performance. See A. A. Tahir, T. Peiris, K.Wijayantha, Chem. Vap. Deposition 18, 107-111 (2012), incorporatedherein by reference in its entirety. The demands of the AACVD techniqueare less stringent as compared to other film deposition methods; it onlyrequires a metal-organic precursor which should be adequately soluble inany organic solvent.

Recently, magnesium dititanate (MgTi₂O₅) electrodes produced by AACVDshowed enhanced PEC water splitting performance and prolongedelectrochemical stability which motivated the development and assessmentof the photoelectroactivity of CaTiO₃. See M. A. Ehsan, R. Naeem, V.McKee, A. S. Hakeem, M. Mazhar Sol. Energy Mater Sol. Cells 161, 328-337(2017), incorporated herein by reference in its entirety. Calciumtitanate (CaTiO₃), again having a perovskite structure, is activelyinvolved in photocatalytic activities. See C. Han, J. Liu, W. Yang, Q.Wu, H. Yang, X. Xue, J. Sol-Gel Sci. Technol. 1-8 (2016), incorporatedherein by reference in its entirety. It has a high reduction potential,which is even higher than that of the well-known TiO₂ photocatalyst. SeeJ. Jang, P. Borse, J. S. Lee, K. Lim, O. Jung, E. Jeong, J. Bae, H. Kim,Bull. Korean Chem. Soc. 32, 95-99 (2011), incorporated herein byreference in its entirety. This has led to significant recent researchdevoted to the photocatalytic performances of CaTiO₃ and its dopedderivatives either for hydrogen production or photodecomposition oforganic molecules. See X.-J. Huang, Y. Xin, H.-y. Wu, F. Ying, Y.-h.Min, W.-s. Li, S.-y. Wang, Z.-j. Wu, Trans. Nonferrous Met. Soc. China26, 464-471 (2016); J. Shi and L. Guo, Pro. Nat. Sci-Mater. 22, 592-615(2012); and H. Zhang, G. Chen, X. He, J. Xu, J. Alloys Compd. 516, 91-95(2012), each incorporated herein by reference in their entirety.Recently a photocatalyst, CaTiO₃/Pt, was designed that demonstratedphotocatalytic hydrogen production under a flow of water and methane.See K. Shimura et al. (2010). The photocatalytic activity of CaTiO₃toward the degradation of organic pollutants is also well established.Yang et al. synthesized the nanocomposite of CaTiO₃-graphene and foundan improved photocatalytic degradation of methyl orange. See T. Xian, H.Yang, Y. Huo, Phys. Scr. 89, 115801 (2014), incorporated herein byreference in its entirety.

Instead of metal titanates, other nanostructured mixed metal oxides havealso shown great promise toward photocatalytic applications. Recently,nanostructured YbVO₄ and YbVO₄/CuWO₄ nanocomposites were successfullysynthesized using a sonochemical method, and their behavior towardsphoto-destruction of methylene blue was evaluated. It has been observedthat due to coupling of CuWO₄ into YbVO₄, photocatalytic and opticalproperties were improved which lead to improve photo-destructionefficiency for methylene blue from 65% to 100%, during 120 minirradiation. See M. Eghbali-Arani, A. Sobhani-Nasab, M.Rahimi-Nasrabadi, F. Ahmadi, S. Pourmasoud, Ultrason. Sonochem. 43,120-135 (2018), incorporated herein by reference in its entirety. In adifferent study, YbVO₄ and nanocomposite YbVO₄/NiWO₄ in a variety ofnano designs and sizes were produced by changing the polymeric cappingagents. The resultant photo-catalysts efficiently performedphoto-degradation of many organic dyes such as rhodamine B, methyleneblue, methyl orange, and phenol red, under visible light. See S.Pourmasoud, A. Sobhani-Nasab, M. Behpour, M. Rahimi-Nasrabadi, F.Ahmadi, J. Mol. Struct. 1157, 607-615 (2018), incorporated herein byreference in its entirety. Moreover, different morphologies of novelZnLaFe₂O₄/NiTiO₃ nanocomposites were fabricated using differentsurfactant agents such as cetrimonium bromide, sodium dodecyl sulfate,polyvinylpyrrolidone, polyvinyl alcohol, and oleic acid through a polyolassistant sol-gel method. This nanocomposite has antibacterial activityagainst Gram-negative Escherichia coli (ATCC 10536) and Gram-positiveStaphylococcus aureus (ATCC 29737). Antibacterial results demonstratethat the nanocomposite significantly reduced the growth rate of E. colibacteria and S. aureus after 120 min. See A. Sobhani-Nasab, Z. Zahraei,M. Akbari, M. Maddahfar, S. M. Hosseinpour-Mashkani, J. Mol. Struct.1139, 430-435 (2017), incorporated herein by reference in its entirety.

In view of the forgoing, one objective of the present invention is thefabrication of CaTiO₃—TiO₂ electrodes by vaporizing a homogenous mixtureof a self-designed Ca-precursor [Ca₂(TFA)₃(OAc))(^(i)PrOH)(H₂O)(THF)₃],and Ti(^(i)Pro)₄ in methanol under AACVD conditions. The deposition ofCaTiO₃—TiO₂ was conducted at three different temperatures of 500, 550,and 600° C. on fluorine doped tin oxide (FTO) coated conductingsubstrates in an air atmosphere. The composite oxide films wereinvestigated by XRD, SEM, EDX, XPS, and UV-VIS spectrophotometry. TheCaTiO₃—TiO₂ films are usable for photocatalytic water splittingapplications.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure relates to acomposite thin film electrode. The composite thin film electrodecomprises a CaTiO₃—TiO₂ layer having an average thickness of 2-12 μm incontact with a substrate. The CaTiO₃—TiO₂ layer comprises crystallineCaTiO₃—TiO₂ particles having an average diameter of 0.2-2.2 μm, and theCaTiO₃—TiO₂ layer comprises 25-87 wt % CaTiO₃ and 13-75 wt % TiO₂, eachrelative to a total weight of the CaTiO₃—TiO₂ layer.

In one embodiment, the CaTiO₃—TiO₂ layer comprises 80-85 wt % CaTiO₃ and15-20 wt % TiO₂, each relative to a total weight of the CaTiO₃—TiO₂layer.

In one embodiment, the crystalline CaTiO₃—TiO₂ particles aresubstantially spherical.

In one embodiment, the TiO₂ is in anatase phase.

In one embodiment, the composite thin film electrode has a direct bandgap value in a range of 2.5-3.5 eV.

In one embodiment, the substrate is a transparent conducting filmselected from the group consisting of ITO, FTO, AZO, GZO, IZO, IZTO,IAZO, IGZO, IGTO, and ATO.

In one embodiment, the substrate has a sheet resistance in a range of1-40 Ωsq⁻¹.

According to a second aspect, the present disclosure relates to a methodof making the composite thin film electrode of the first aspect. Thismethod involves contacting an aerosol with a substrate to deposit acrystalline CaTiO₃—TiO₂ composite layer on the substrate to form thecomposite thin film electrode. The aerosol comprises a carrier gas and acalcium complex and a titanium complex dissolved in a solvent, and thesubstrate has a temperature in a range of 400-650° C. during thecontacting.

In one embodiment, the calcium complex comprises trifluoroacetateligands, acetate ligands, isopropanol ligands, and tetrahydrofuranligands.

In a further embodiment, the calcium complex has a formula[Ca₂(TFA)₃(OAc))(^(i)PrOH)(H₂O)(THF)₃].

In one embodiment, before the contacting, the aerosol consistsessentially of the carrier gas, the calcium complex, the titaniumcomplex, and the solvent.

In one embodiment, the calcium complex and the solvent are present inthe aerosol at a weight ratio of 1:1,000-1:5.

In one embodiment, the titanium complex and the solvent are present inthe aerosol at a weight ratio of 1:10,000-1:5.

In one embodiment, the aerosol is contacted with the substrate for atime period of 10-120 min.

In one embodiment, during the contacting, the carrier gas has a flowrate in a range of 20-250 mL/min.

According to a third aspect, the present disclosure relates to anelectrochemical cell, which is made of the composite thin film electrodeof the first aspect, a counter electrode, and an electrolyte solution incontact with both electrodes.

In one embodiment, the electrolyte solution comprises water and aninorganic base having a concentration of 0.5-1.5 M.

In one embodiment, the composite thin film electrode has a currentdensity of 0.45-0.8 mA/cm² when the electrodes are subjected to a biaspotential of 0.6-0.8 V and an illumination of 80-150 mW/cm².

In one embodiment, the composite thin film electrode has a chargetransfer resistance in a range of 200-400Ω when the electrodes aresubjected to an illumination of 80-150 mW/cm².

According to a fourth aspect, the present disclosure relates to a methodof photocatalytic water splitting. This involves irradiating theelectrochemical cell of the third aspect with sunlight.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of the AACVD setup used for synthesisof CaTiO₃—TiO₂ composite films.

FIG. 2 shows a section of the polymeric chain structure.

FIG. 3 shows TGA (black) and DTG (dotted red) profiles indicatingpyrolysis of complex (1) against the increasing temperature under aninert nitrogen atmosphere and temperature ramping of 10° C. min⁻¹.

FIG. 4A shows XRD peak patterns of CaTiO₃—TiO₂ composite oxide filmsdeposited on FTO glass substrates at different temperatures via AACVD.

FIG. 4B shows the crystalline proportion of the phases involved incomposite films formed at 500° C.

FIG. 4C shows the crystalline proportion of the phases involved incomposite films formed at 550° C.

FIG. 4D shows the crystalline proportion of the phases involved incomposite films formed at 600° C.

FIG. 5A shows a low resolution (10K×) surface FESEM image of aCaTiO₃—TiO₂ thin film deposited on an FTO glass substrate at atemperature of 500° C.

FIG. 5B shows a high resolution (50K×) surface FESEM image of aCaTiO₃—TiO₂ thin film deposited on an FTO glass substrate at atemperature of 500° C.

FIG. 5C shows a cross-sectional FESEM image of a CaTiO₃—TiO₂ thin filmdeposited on an FTO glass substrate at a temperature of 500° C.

FIG. 6A shows a low resolution (10K×) surface FESEM image of aCaTiO₃—TiO₂ thin film deposited on an FTO glass substrate at atemperature of 550° C.

FIG. 6B shows a high resolution (50K×) surface FESEM image of aCaTiO₃—TiO₂ thin film deposited on an FTO glass substrate at atemperature of 550° C.

FIG. 6C shows a cross-sectional FESEM image of a CaTiO₃—TiO₂ thin filmdeposited on an FTO glass substrate at a temperature of 550° C.

FIG. 7A shows a low resolution (10K×) surface FESEM image of aCaTiO₃—TiO₂ thin film deposited on an FTO glass substrate at atemperature of 600° C.

FIG. 7B shows a high resolution (50K×) surface FESEM image of aCaTiO₃—TiO₂ thin film deposited on an FTO glass substrate at atemperature of 600° C.

FIG. 7C shows a cross-sectional FESEM image of a CaTiO₃—TiO₂ thin filmdeposited on an FTO glass substrate at a temperature of 600° C.

FIG. 8A shows the EDX spectra recorded from an area of CaTiO₃—TiO₂ filmdeposited at 500° C. on FTO substrate.

FIG. 8B shows an FESEM of the area of FIG. 8A.

FIG. 8C shows the EDX spectra recorded from an area of CaTiO₃—TiO₂ filmdeposited at 500° C. on FTO substrate.

FIG. 8D shows an FESEM of the area of FIG. 8C.

FIG. 8E shows the EDX spectra recorded from an area of CaTiO₃—TiO₂ filmdeposited at 500° C. on FTO substrate.

FIG. 8F shows an FESEM of the area of FIG. 8E.

FIG. 8G shows the EDX spectra recorded from an area of CaTiO₃—TiO₂ filmdeposited at 500° C. on FTO substrate.

FIG. 8H shows an FESEM of the area of FIG. 8G.

FIG. 9A shows the EDX spectra recorded from an area of CaTiO₃—TiO₂ filmdeposited at 550° C. on FTO substrate.

FIG. 9B shows an FESEM of the area of FIG. 9A.

FIG. 9C shows the EDX spectra recorded from an area of CaTiO₃—TiO₂ filmdeposited at 550° C. on FTO substrate.

FIG. 9D shows an FESEM of the area of FIG. 9C.

FIG. 9E shows the EDX spectra recorded from an area of CaTiO₃—TiO₂ filmdeposited at 550° C. on FTO substrate.

FIG. 9F shows an FESEM of the area of FIG. 9E.

FIG. 9G shows the EDX spectra recorded from an area of CaTiO₃—TiO₂ filmdeposited at 550° C. on FTO substrate.

FIG. 9H shows an FESEM of the area of FIG. 9G.

FIG. 10A shows the EDX spectra recorded from an area of CaTiO₃—TiO₂ filmdeposited at 600° C. on FTO substrate.

FIG. 10B shows an FESEM of the area of FIG. 10A.

FIG. 10C shows the EDX spectra recorded from an area of CaTiO₃—TiO₂ filmdeposited at 600° C. on FTO substrate.

FIG. 10D shows an FESEM of the area of FIG. 10C.

FIG. 10E shows the EDX spectra recorded from an area of CaTiO₃—TiO₂ filmdeposited at 600° C. on FTO substrate.

FIG. 10F shows an FESEM of the area of FIG. 10E.

FIG. 10G shows the EDX spectra recorded from an area of CaTiO₃—TiO₂ filmdeposited at 600° C. on FTO substrate.

FIG. 10H shows an FESEM of the area of FIG. 10G.

FIG. 11A is an FESEM image with overlaid EDX elemental map showing thecombined distribution of Ca, Ti, and G atoms in a CaTiO₃—TiO₂ compositefilm deposited at 500° C.

FIG. 11B is the EDX elemental map of FIG. 11A showing only thedistribution of Ti atoms.

FIG. 11C is the EDX elemental map of FIG. 11A showing only thedistribution of Ca atoms.

FIG. 11D is the EDX elemental map of FIG. 11A showing only thedistribution of O atoms.

FIG. 12A is an FESEM image with overlaid EDX elemental map showing thecombined distribution of Ca, Ti, and O atoms in a CaTiO₃—TiO₂ compositefilm deposited at 550° C.

FIG. 12B is the EDX elemental map of FIG. 12A showing only thedistribution of Ti atoms.

FIG. 12C is the EDX elemental map of FIG. 12A showing only thedistribution of Ca atoms.

FIG. 12D is the EDX elemental map of FIG. 12A showing only thedistribution of O atoms.

FIG. 13A is an FESEM image with overlaid EDX elemental map showing thecombined distribution of Ca, Ti, and O atoms in a CaTiO₃—TiO₂ compositefilm deposited at 600° C.

FIG. 13B is the EDX elemental map of FIG. 13A showing only thedistribution of Ti atoms.

FIG. 13C is the EDX elemental map of FIG. 13A showing only thedistribution of Ca atoms.

FIG. 13D is the EDX elemental map of FIG. 13A showing only thedistribution of O atoms.

FIG. 14A is an XPS survey scan spectrum of CaTiO₃—TiO₂ composite filmdeposited at 600° C.

FIG. 14B is a high resolution XPS spectrum of the film of FIG. 14A forCa 2p.

FIG. 14C is a high resolution XPS spectrum of the film of FIG. 14A forTi 2p.

FIG. 14D is a high resolution XPS spectrum of the film of FIG. 14A for O1s.

FIG. 15A is a UV-Visible absorption spectrum of CaTiO₃—TiO₂ composite,deposited at 600° C. by AACVD.

FIG. 15B the corresponding Tauc's plot of (αhν)² vs. E/eV for thespectrum of FIG. 15A.

FIG. 16A shows LSV curves for CaTiO₃—TiO₂ electrodes deposited at 500,550 and 600° C. in 1M NaOH under dark and simulated AM 1.5 illuminationof 100 mWcm⁻² FIG. 16B shows transient photocurrent responses recordedfor CaTiO₃—TiO₂ electrodes deposited at 500, 550 and 600° C. in 1M NaOHunder dark and simulated AM 1.5 illumination of 100 mWcm⁻².

FIG. 17A shows EIS Nyquist plots for the CaTiO₃—TiO₂ films fabricated atdifferent temperatures and in light and dark conditions.

FIG. 17B shows Bode phase plots observed for the CaTiO₃—TiO₂ filmsfabricated at different temperatures using a frequency range of 0.1Hz-10 kHz both in dark and light conditions.

FIG. 18 shows a schematic representation of band positions and thespatial separation of light-generated inter-particle charges duringphotocatalytic activation of CaTiO₃—TiO₂ with a negative shift in itsFermi level.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the disclosure are shown.

The present disclosure will be better understood with reference to thefollowing definitions. As used herein, the words “a” and “an” and thelike carry the meaning of “one or more.” Within the description of thisdisclosure, where a numerical limit or range is stated, the endpointsare included unless stated otherwise. It will be further understood thatthe terms “comprises” and/or “comprising,” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

As used herein, the words “about,” “approximately,” or “substantiallysimilar” may be used when describing magnitude and/or position toindicate that the value and/or position described is within a reasonableexpected range of values and/or positions. For example, a numeric valuemay have a value that is +/−0.1% of the stated value (or range ofvalues), +/−1% of the stated value (or range of values), +/−2% of thestated value (or range of values), +/−5% of the stated value (or rangeof values), +/−10% of the stated value (or range of values), +/−15% ofthe stated value (or range of values), or +/−20% of the stated value (orrange of values). Within the description of this disclosure, where anumerical limit or range is stated, the endpoints are included unlessstated otherwise. Also, all values and subranges within a numericallimit or range are specifically included as if explicitly written out.

As used herein, “compound” and “complex” are each intended to refer to achemical entity, whether as a solid, liquid, or gas, and whether in acrude mixture or isolated and purified.

As used herein, “composite” refers to a combination of two or moredistinct constituent materials into one. The individual components, onan atomic level, remain separate and distinct within the finishedstructure. The materials may have different physical or chemicalproperties, that when combined, produce a material with characteristicsdifferent from the original components. In some embodiments, a compositemay have at least two constituent materials that comprise the sameempirical formula but are distinguished by different densities, crystalphases, or a lack of a crystal phase (i.e. an amorphous phase).

The present disclosure is intended to include all hydration states of agiven compound or formula, unless otherwise noted or when heating amaterial. For example, Ni(NO₃)₂ includes anhydrous Ni(NO₃)₂,Ni(NO₃)₂.6H₂O, and any other hydrated forms or mixtures. CuCl₂ includesboth anhydrous CuCl₂ and CuCl₂.2H₂O.

In addition, the present disclosure is intended to include all isotopesof atoms occurring in the present compounds and complexes. Isotopesinclude those atoms having the same atomic number but different massnumbers. By way of general example, and without limitation, isotopes ofhydrogen include deuterium and tritium. Isotopes of carbon include ¹³Cand ¹⁴C. Isotopes of nitrogen include ¹⁴N and ⁵N. Isotopes of oxygeninclude ¹⁶O, ¹⁷O, and ¹⁸O. Isotopes of titanium include ⁴⁴Ti, ⁴⁶Ti,⁴⁷Ti, ⁴⁸Ti, ⁴⁹Ti, and ⁵⁰Ti. Isotopes of calcium include ⁴⁰Ca, ⁴¹Ca,⁴²Ca, ⁴³Ca, ⁴⁴Ca, ⁴⁵Ca, ⁴⁶Ca, ⁴⁷Ca, and ⁴⁸Ca. Isotopically-labeledcompounds of the disclosure may generally be prepared by conventionaltechniques known to those skilled in the art or by processes analogousto those described herein, using an appropriate isotopically-labeledreagent in place of the non-labeled reagent otherwise employed.

As defined here, an aerosol is a suspension of solid or liquid particlesin a gas. An aerosol includes both the particles and the suspending gas.Primary aerosols contain particles introduced directly into the gas,while secondary aerosols form through gas-to-particle conversion. Thereare several measures of aerosol concentration. Environmental science andhealth fields often use the mass concentration (M), defined as the massof particulate matter per unit volume with units such as μg/m³. Alsocommonly used is the number concentration (N), the number of particlesper unit volume with units such as number/m³ or number/cm³. The size ofparticles has a major influence on their properties, and the aerosolparticle radius or diameter (d_(p)) is a key property used tocharacterize aerosols. Aerosols vary in their dispersity. A monodisperseaerosol, producible in the laboratory, contains particles of uniformsize. Most aerosols, however, as polydisperse colloidal systems, exhibita range of particle sizes. Liquid droplets are almost always nearlyspherical, but scientists use an equivalent diameter to characterize theproperties of various shapes of solid particles, some very irregular.The equivalent diameter is the diameter of a spherical particle with thesame value of some physical property as the irregular particle. Theequivalent volume diameter (d_(e)) is defined as the diameter of asphere of the same volume as that of the irregular particle. Alsocommonly used is the aerodynamic diameter. The aerodynamic diameter ofan irregular particle is defined as the diameter of the sphericalparticle with a density of 1000 kg/m³ and the same settling velocity asthe irregular particle.

As defined here, an electrode is an electrically conductive materialcomprising a metal and is used to establish electrical contact with anonmetallic part of a circuit. An “electrically-conductive material” asdefined here is a substance with an electrical resistivity of at most10⁻⁶ Ω·m, preferably at most 10⁻⁷ Ω·m, more preferably at most 10⁻⁸ Ω·mat a temperature of 20-25° C. The electrically-conductive materialcomprise platinum-iridium alloy, iridium, titanium, titanium alloy,stainless steel, gold, cobalt alloy, copper, aluminum, tin, iron, and/orsome other metal.

According to a first aspect, the present disclosure relates to acomposite thin film electrode. The composite thin film electrode mayalso be called a photoactive composite thin film electrode, aCaTiO₃—TiO₂ composite thin film electrode, or a photoactive CaTiO₃—TiO₂composite thin film electrode. The composite thin film electrodecomprises a CaTiO₃—TiO₂ layer in contact with a substrate.

The CaTiO₃—TiO₂ layer comprises or is in the form of crystallineCaTiO₃—TiO₂ particles. In one embodiment, at least 80 wt %, preferablyat least 85 wt %, more preferably at least 95 wt % of the CaTiO₃—TiO₂ iscrystalline CaTiO₃ and crystalline TiO₂. Where the CaTiO₃—TiO₂ layerand/or particles comprise less than 100 wt % crystalline CaTiO₃ andcrystalline TiO₂, the remaining CaTiO₃ and/or TiO₂ may be in anamorphous (non-crystalline) phase.

In one embodiment, the layer has an average thickness of 2-12 μm,preferably 5-11 μm, more preferably 7-10 μm, or about 9 μm. However, insome embodiments, the layer may have a thickness of less than 2 μm orgreater than 12 μm. In one embodiment, the thickness of the layer mayvary from location to location on the electrode by 1-30%, preferably5-20%, relative to an average thickness of the layer.

In one embodiment, the CaTiO₃—TiO₂ layer comprises 25-87 wt % CaTiO₃ and13-75 wt % TiO₂, each relative to a total weight of the CaTiO₃—TiO₂layer. In other embodiments, the CaTiO₃—TiO₂ layer comprises 45-86.5 wt% CaTiO₃, preferably 65-86.0 wt % CaTiO₃, more preferably 75-85.5 wt %CaTiO₃, and 13.5-55 wt % TiO₂, preferably 14.0-35 wt % TiO₂, morepreferably 14.5-25 wt % TiO₂, each relative to a total weight of theCaTiO₃—TiO₂ layer.

In one embodiment, the CaTiO₃—TiO₂ layer comprises 80-85 wt % CaTiO₃,preferably 81-84 wt % CaTiO₃, or about 83 wt % CaTiO₃, and 15-20 wt %TiO₂, preferably 16-19 wt % TiO₂, or about 17 wt % TiO₂, each relativeto a total weight of the CaTiO₃—TiO₂ layer.

In one embodiment, the CaTiO₃—TiO₂ layer consists essentially of CaTiO₃and TiO₂, meaning that the CaTiO₃—TiO₂ comprises at least 99.5 wt %,preferably at least 99.8 wt % CaTiO₃ and TiO₂, relative to a totalweight of the CaTiO₃—TiO₂ layer.

The CaTiO₃—TiO₂ particles may leave pores or open spaces when packed inthe CaTiO₃—TiO₂ layer. In one embodiment, the CaTiO₃—TiO₂ layer may havea porosity in a range of 10-70%, preferably 20-60%. In a relatedembodiment, the CaTiO₃—TiO₂ layer may have a surface area per massCaTiO₃—TiO₂ of 80-350 m²/g, preferably 100-250 m²/g, even morepreferably 120-220 m²/g.

In one embodiment, the CaTiO₃—TiO₂ particles are substantiallyspherical, meaning that the distance from the particle centroid toanywhere on the outer surface varies by less than 30%, preferably lessthan 20%, or less than 10%. In alternative embodiments, one or moreCaTiO₃—TiO₂ particles may be shaped like cylinders, boxes, spikes,flakes, plates, ellipsoids, toroids, stars, ribbons, discs, rods,granules, prisms, cones, flakes, platelets, sheets, or some other shape.In one embodiment, the individual CaTiO₃—TiO₂ particles comprise bothCaTiO₃ and TiO₂. In an alternative embodiment, some CaTiO₃—TiO₂particles may comprise some particles with a higher concentration ofCaTiO₃ than the bulk average and may comprise other particles withhigher concentration of TiO₂ than the bulk average. In one embodiment,individual CaTiO₃—TiO₂ particles may be considered nanocomposites, wherethe particle has regions of CaTiO₃ and TiO₂ interspersed throughout theparticle. These regions maybe considered nanodomains, and may have anaverage diameter in a range of 10-100 nm, 15-90 nm, or 20-80 nm.

In one embodiment, the CaTiO₃—TiO₂ particles may have an averagediameter or average longest dimension in a range of 0.2-2.2 μm,preferably 0.4-1.5 μm, more preferably 0.5-1.0 μm, or about 0.75 μm.However, in alternative embodiments, the CaTiO₃—TiO₂ particles may havean average diameter or average longest dimension of less than 0.2 μm orgreater than 2.2 μm.

In one embodiment, the CaTiO₃—TiO₂ particles may have an average Wadellsphericity value in a range of 0.3 to 0.9, or 0.3 to 0.8. The Wadellsphericity of a particle is defined by the ratio of the surface area ofa sphere (which has the same volume as the given particle) to thesurface area of the particle. The values of Wadell sphericity range from0 to 1, where a value of 1 is a perfect sphere, and particles becomeless spherical as their sphericity approaches a value of 0. The Wadellsphericity may be approximated by

${\Psi \approx \left( \frac{bc}{a^{2}} \right)^{1/3}},$

where a, b, and c are the lengths of the long, intermediate, and shortaxes, respectively of an individual particle.

In one embodiment, the CaTiO₃—TiO₂ particles are monodisperse, having acoefficient of variation or relative standard deviation, expressed as apercentage and defined as the ratio of the particle diameter standarddeviation (σ) to the particle diameter mean (μ), multiplied by 100%, ofless than 25%, preferably less than 10%, preferably less than 8%,preferably less than 6%, preferably less than 5%. In a preferredembodiment, the CaTiO₃—TiO₂ particles are monodisperse having a particlediameter distribution ranging from 80% of the average particle diameterto 120% of the average particle diameter, preferably 85-115%, preferably90-110% of the average particle diameter. In another embodiment, theCaTiO₃—TiO₂ particles are not monodisperse.

CaTiO₃ may be called calcium titanate or perovskite. TiO₂ exists inthree crystalline phases: anatase, rutile, and brookite, which may bedetermined by XRD patterns. Mixtures of TiO₂ polymorphs may havedifferent photocatalytic activities compared to those of the purephases, because of variations in electron-hole separation properties. Inone embodiment, the TiO₂ of the composite thin film electrode may beprimarily anatase, for instance, at least 70 wt %, preferably at least80 wt %, more preferably at least 90 wt %, even more preferably at least95 wt % or about 100 wt % of the total TiO₂ weight is anatase phaseTiO₂. In one embodiment, the TiO₂ comprises at least 90% anatase phaseTiO₂. In one embodiment, the TiO₂ may be multiphasic. The term“multiphasic,” as used herein, refers to a compound comprising two ormore types of amorphous and/or crystalline phases. Biphasic compoundsand triphasic compounds may be referred to as multiphasic compounds. Inembodiments where the TiO₂ of the composite thin film electrodecomprises less than 100 wt % anatase phase, the other phases may berutile, brookite, or amorphous TiO₂. In a preferred embodiment, the TiO₂is in anatase phase.

In one embodiment, the composite thin film electrode has a direct bandgap value in a range of 2.5-3.5 eV, preferably 2.6-3.4 eV, morepreferably 2.7-3.3 eV even more preferably 2.8-3.2 eV, or about 3.0 eV.

In one embodiment, the substrate is a transparent conducting filmselected from the group consisting of ITO (indium tin oxide), FTO(fluorine-doped tin oxide), AZO (aluminum-doped zinc oxide), GZO(gallium-doped zinc oxide), IZO (indium zinc oxide), IZTO (indium zinctin oxide), IAZO (indium aluminum zinc oxide), IGZO (indium gallium zincoxide), IGTO (indium gallium tin oxide), and ATO (antimony tin oxide).In other embodiments, transparent conducting polymers (such as PEDOT) orcarbon nanotubes may be used with or in place of the compoundspreviously mentioned. In a preferred embodiment, the substrate is FTO.The transparent conducting film may have an average thickness of 1 μm-1mm, preferably 10 μm-900 μm, more preferably 200 μm-800 μm, or about 600μm. Alternatively, the transparent conducting film may have an averagethickness of 500 nm-200 μm, preferably 1 μm-100 μm, more preferably 10μm-50 μm. However, in some embodiments, the conductive layers may havean average thickness of less than 500 nm. For instance, the conductivelayers may have an average thickness of 50-500 nm, 80-300 nm, or 100-250nm. Preferably the transparent conducting film is attached to anadditional support, such as a glass slide. However, in otherembodiments, the substrate may be glass, quartz, ceramic, a metal, acomposite material, or a polymeric material having temperatureresistance at least up to the temperature of the substrate heating.Where the substrate comprises glass, the glass may beboro-aluminosilicate glass, sodium borosilicate glass, fused-silicaglass, soda lime glass, or some other type of glass.

In one embodiment, the substrate has a sheet resistance in a range of1-40 Ωsq⁻¹, preferably 2-20 Ωsq⁻¹, more preferably 4-12 Ωsq⁻¹, or about8 Ωsq⁻¹. Preferably, the CaTiO₃—TiO₂ particles in contact with thesubstrate form an electrically-conductive material with the conductivelayer. An “electrically-conductive material” as defined here is asubstance with an electrical resistivity of at most 10⁻⁶ Ω·m, preferablyat most 10⁻⁷ Ω·m, more preferably at most 10⁻⁸ Ω·m at a temperature of20-25° C.

According to a second aspect, the present disclosure relates to a methodof making the composite thin film electrode of the first aspect. Thismethod involves contacting an aerosol with a substrate to deposit acrystalline CaTiO₃—TiO₂ composite layer on the substrate to form thecomposite thin film electrode. As described here, “contacting an aerosolwith a substrate” is considered to be synonymous with “contacting asubstrate with an aerosol.” Both phrases mean that the substrate isexposed to the aerosol, so that a portion of the suspended particles ofthe aerosol directly contact the substrate. Contacting may also beconsidered equivalent to “introducing” or “depositing,” such as“depositing an aerosol on a substrate.” In one embodiment, thecontacting may be considered aerosol-assisted chemical vapor deposition(AACVD). In one embodiment, the method of making the composite thin filmelectrode may be considered a one-step method, as the formation of theCaTiO₃—TiO₂ composite layer takes place immediately following and/orduring the contacting of the aerosol with the substrate.

In one embodiment, the temperature of the substrate during thecontacting is in a range of 400-650° C., preferably 450-640° C., morepreferably 500-630° C., even more preferably 570-620° C., or about 600°C. In one embodiment, the temperature of the substrate during thecontacting never reaches a temperature of greater than 650° C.,preferably 625° C., more preferably 610° C.

The aerosol comprises a carrier gas, a calcium complex, a titaniumcomplex, and a solvent. In one embodiment, the aerosol consists of, orconsists essentially of, a carrier gas, a calcium complex, a titaniumcomplex, and a solvent before the contacting, preferably immediatelybefore the contacting. Preferably, the calcium complex and titaniumcomplex are dissolved or dispersed in the solvent. In some embodiments,the calcium complex and titanium complex are dissolved in the sameaerosol droplets. In other embodiments, some aerosol droplets mayconsist of the calcium complex and solvent, and other aerosol dropletsmay consist of titanium complex and solvent.

In one embodiment, the calcium complex comprises trifluoroacetate (TFA)ligands, acetate ligands (OAc), isopropanol (^(i)PrOH) ligands,tetrahydrofuran (TIF) ligands, and water (H₂O) ligands. In oneembodiment, a molar ratio of TFA ligands to Ca in the calcium complex isin a range of 1:1-2:1, or about 3:2. In one embodiment, a molar ratio ofacetate ligands to Ca in the calcium complex is in a range of 1:4-2:1,or about 1:2. In one embodiment, a molar ratio of isopropanol ligands tocalcium is in a range of 1:4-2:1, or about 1:2. In one embodiment, amolar ratio of H₂O ligands to calcium is in a range of 1:4-2:1, or about1:2. In one embodiment, a molar ratio of THF ligands to calcium is in arange of 1:4-2:1, or about 3:2. In a further embodiment, the calciumcomplex has a formula [Ca₂(TFA)₃(OAc))(^(i)PrOH)(H₂O)(THF)₃], which mayalso be represented as C₁₅H₂₁Ca₂F₉O₁₁.2(C₄H₈O). In alternativeembodiments, the calcium complex may not comprise one or more of theligands trifluoroacetate, acetate, isopropanol, tetrahydrofuran, orwater, and in other embodiments, one or more ligands may be substitutedwith other ligands, such as ethanol. In other alternative embodiments,calcium salts such as CaO, CaCl₂, or CaC₂O₄ may be used in place of orin addition to the calcium complex. In one embodiment, the calciumcomplex and the solvent are present in the aerosol at a calcium complexto solvent weight ratio of 1:1000-1:5, preferably 1:500-1:10, morepreferably 1:200-1:15, even more preferably 1:100-1:20, or about 1:23.

In one embodiment, the titanium complex may be TiB₂, TiBr, TiC, TiCl₄,Ti(ClO₄)₄, TiF₄, H₂TiF₆, TiH₄, TiI₄, Ti(NMe₂)₄, Ti(NO₃)₄, TiO₂, H₄TiO₄,Ti₄(OCH₂CH₃)₁₆, Ti(OCH₂CH₂CH₂CH₃)₄, KTiOPO₄, NiO Sb₂O₃.2OTiO₂, TiS₂,TiSe₂, TiSi₂, titanium(IV) isopropoxide, or some other titaniumcompound. Preferably the titanium complex is titanium(IV) isopropoxide,and may be abbreviated as Ti(^(i)Pro)₄. In one embodiment, the titaniumcomplex and the solvent are present in the aerosol at a titanium complexto solvent weight ratio of 1:10,000-1:5, preferably 1:1,000-1:20, morepreferably 1:800-1:100, even more preferably 1:500-1:150, or about1:206.

In an alternative embodiment, rather than a calcium complex and atitanium complex existing as separate molecules, a single moleculecomprising both calcium and titanium may be used.

In one embodiment, the carrier gas is N₂, He, compressed air, and/or Ar.Preferably the carrier gas is compressed air.

In one embodiment, the solvent may be toluene, tetrahydrofuran, aceticacid, acetone, acetonitrile, butanol, dichloromethane, chloroform,chlorobenzene, dichloroethane, diethylene glycol, diethyl ether,dimethoxy-ethane, dimethyl-formamide, dimethyl sulfoxide, ethanol, ethylacetate, ethylene glycol, heptane, hexamethylphosphoramide,hexamethylphosphorous triamide, methanol, methyl t-butyl ether,methylene chloride, pentane, cyclopentane, hexane, cyclohexane, benzene,dioxane, propanol, isopropyl alcohol, pyridine, triethyl amine,propandiol-1,2-carbonate, ethylene carbonate, propylene carbonate,nitrobenzene, formamide, γ-butyrolactone, benzyl alcohol,n-methyl-2-pyrrolidone, acetophenone, benzonitrile, valeronitrile,3-methoxy propionitrile, dimethyl sulfate, aniline, n-methylformamide,phenol, 1,2-dichlorobenzene, tri-n-butyl phosphate, ethylene sulfate,benzenethiol, dimethyl acetamide, N,N-dimethylethaneamide,3-methoxypropionnitrile, diglyme, cyclohexanol, bromobenzene,cyclohexanone, anisole, diethylformamide, 1-hexanethiol, ethylchloroacetate, 1-dodecanthiol, di-n-butylether, dibutyl ether, aceticanhydride, m-xylene, o-xylene, p-xylene, morpholine, diisopropyletheramine, diethyl carbonate, 1-pentandiol, n-butyl acetate, and/or1-hexadecanthiol. In one embodiment, the solvent comprises pyridine,N,N-dimethylformamide (DMF), N,N-dimethylacetamide, N-methyl pyrrolidone(NMP), hexamethylphosphoramide (HMPA), dimethyl sulfoxide (DMSO),acetonitrile, tetrahydrofuran (THF), 1,4-dioxane, dichloromethane,chloroform, carbon tetrachloride, dichloroethane, acetone, ethylacetate, pentane, hexane, decalin, dioxane, benzene, toluene, xylene,o-dichlorobenzene, diethyl ether, methyl t-butyl ether, methanol,ethanol, ethylene glycol, isopropanol, propanol, and/or n-butanol. In apreferred embodiment, the solvent is acetone, methanol, ethanol, and/orisopropanol. More preferably the solvent is methanol, and in anotherembodiment, the solvent consists essentially of methanol.

In one embodiment, the solvent may comprise water. The water used as asolvent or for other purposes may be tap water, distilled water,bidistilled water, deionized water, deionized distilled water, reverseosmosis water, and/or some other water. In one embodiment the water isbidistilled or treated with reverse osmosis to eliminate trace metals.Preferably the water is bidistilled, deionized, deionized distilled, orreverse osmosis water, and at 25° C. has a conductivity of less than 10μS·cm⁻¹, preferably less than 1 μS·cm⁻¹; a resistivity of greater than0.1 MΩ·cm, preferably greater than 1 MΩ·cm, more preferably greater than10 MΩ·cm; a total solid concentration of less than 5 mg/kg, preferablyless than 1 mg/kg; and a total organic carbon concentration of less than1000 μg/L, preferably less than 200 μg/L, more preferably less than 50μg/L.

Preferably the solvent and the calcium complex and/or titanium complexare able to form an appropriately soluble solution that can be dispersedin the carrier gas as aerosol particles. For instance, the calciumcomplex and/or titanium complex may first be dissolved in a volume ofsolvent, and then pumped through a jet nozzle in order to create anaerosol mist. In other embodiments, the mist may be generated by apiezoelectric ultrasonic generator. Other nebulizers and nebulizerarrangements may also be used, such as concentric nebulizers, cross-flownebulizers, entrained nebulizers, V-groove nebulizers, parallel pathnebulizers, enhanced parallel path nebulizers, flow blurring nebulizers,and piezoelectric vibrating mesh nebulizers. In one embodiment, themixtures of the calcium complex and solvent, and the titanium complexand solvent, are introduced as separate aerosols, for instance, producedby separate nozzles or nebulizers. Preferably, however, the calciumcomplex and titanium complex are mixed together in the same solventprior to producing the aerosol.

In one embodiment, the aerosol may have a mass concentration M, of 10μg/m³-1,000 mg/m³, preferably 50 μg/m³-1,000 μg/m³. In one embodiment,the aerosol may have a number concentration N, in a range of 10³-10⁶,preferably 10⁴-10⁵ cm⁻³. In other embodiments, the aerosol may have anumber concentration of less than 10³ or greater than 10⁶. The aerosolparticles or droplets may have an equivalent volume diameter (d_(e)) ina range of 20 nm-100 μm, preferably 0.5-70 μm, more preferably 1-50 μm,though in some embodiments, aerosol particles or droplets may have anaverage diameter of smaller than 0.2 μm or larger than 100 μm.

In one embodiment, the aerosol and substrate do not comprise or contacthydrogen gas or a reducing agent during the contacting and/ordepositing. In a related embodiment, the aerosol and substrate do notcomprise or contact hydrogen gas or a reducing agent immediately priorto the contacting and/or depositing. In one embodiment, the reactionchamber where the depositing takes place is essentially free of hydrogengas and a reducing agent immediately prior to the contacting. In oneembodiment, an intermediate reducing agent is created during thecontacting.

In a related embodiment, before the contacting and/or depositing, theaerosol consists essentially of the carrier gas, the solvent, thetitanium complex, and the calcium complex, meaning that at least 99.9 wt%, preferably at least 99.99 wt %, or 100 wt % of the aerosol is carriergas, solvent, titanium complex, or calcium complex, relative to a totalweight of the aerosol.

In one embodiment, the aerosol is contacted with the substrate for atime period of 10-120 min, preferably 20-100 min, more preferably 30-90min, even more preferably 45-75 min, or about 60 min. However, in someembodiments, the aerosol may be contacted with the substrate for a timeperiod of less than 10 min or greater than 120 min.

In one embodiment, during the contacting of the aerosol, the carrier gashas a flow rate in a range of 0.1 to 10 mL/s, preferably 0.5 to 7.5mL/s, more preferably 1.5 to 5.0 mL/s, even more preferably 2.0-4.0mL/s, or about 2.5 mL/s (equivalently 150 cm³/min).

However, in some embodiments, the carrier gas flow rate may be less than0.1 mL/s or greater than 10 mL/s. Preferably, the aerosol has a flowrate similar to the carrier gas, with the exception of the portion ofaerosol that gets deposited on the substrate. In one embodiment, theaerosol may enter the chamber and the flow rate may be stopped, so thata portion of aerosol may settle on the substrate.

In one embodiment, the aerosol is contacted with the substrate in areaction chamber. The flow of the carrier gas and aerosol may have a gashourly space velocity in a range of 10-1,000 h⁻¹, preferably 50-500 h⁻¹,more preferably 100-130 h⁻¹.

The contacting and/or introducing may take place within a closed chamberor reactor, and the aerosol may be generated by dispersing a solution ofthe calcium complex and/or titanium complex and solvent. In oneembodiment, this dispersing may be increased by fans, jets, or pumps.However, in another embodiment, an aerosol may be formed in a closedchamber with a substrate where the aerosol particles are allowed todiffuse towards or settle on the substrate. The substrate may have anarea in a range of 0.5-4 cm², preferably 1.0-3 cm², more preferably1.5-3 cm². In one embodiment, the closed chamber or reactor may have alength of 10-100 cm, preferably 12-30 cm, and a diameter or width of1-10 cm, preferably 2-5 cm. In other embodiments, the closed chamber orreactor may have an interior volume of 0.2-100 L, preferably 0.3-25 L,more preferably 0.5-10 L. In one embodiment, the closed chamber orreactor may comprise a cylindrical glass vessel, such as a glass tube.

Being in a closed chamber, the interior pressure of the chamber (andthus the pressure of the aerosol) may be controlled. The pressure may bepractically unlimited, but need not be an underpressure or anoverpressure. Preferably the chamber and substrate are heated and heldat a temperature prior to the contacting, so that the temperature maystabilize. The chamber and substrate may be heated for a time period of5 min-1 hr, preferably 10-20 min prior to the contacting.

During the contacting of the aerosol, the CaTiO₃—TiO₂ composite layermay form at a rate of 0.1 to 20, 0.2 to 18, 0.4 to 16, 0.5 to 14, 0.6 to12, 0.7 to 10, 0.8 to 9, 3 to 15, 1.0 to 8, 1.5 to 5, or 2 to 3 nm/s,and/or at least 0.01, 0.05, 0.1, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0, 1.5,1.75, 2, 2.5, 3.33, 3.5, 4, 4.5, 5, 6.5, 7, 7.5, 7.75, 8, 8.25, 8.5,8.75, 9, or 10 nm/s.

In one embodiment, the method of making the composite thin filmelectrode may further comprise a step of cooling the composite thin filmelectrode after the contacting. The composite thin film electrode may becooled to a temperature between 10 to 45° C., 20 to 40° C., or 25 to 35°C. under an inert gas (such as N₂ or Ar) over a time period of 0.5 to 5h, 0.75 to 4 h, 1 to 3 h, 1.25 to 2.5 h, or 1.5 to 2 h.

In one embodiment, the method of making the composite thin filmelectrode may further comprise a step of preparing the calcium complexbefore the contacting. The calcium complex may be synthesized by methodsdescribed herein, or by mixing Ca(OAc)₂, Ti(^(i)Pro)₄, andtrifluoroacetic acid in THF to form a mixture. The mixture may bestirred for 0.5-6 h, preferably 1-3 h under an inert atmosphere of N₂ orAr gas. The reaction mixture may then be dried to yield the calciumcomplex, or alternatively, the reaction mixture may be dried,resuspended in THF, and then dried a second time to yield the calciumcomplex.

An example AACVD setup is illustrated in FIG. 1 . Here, a container ofthe Ca—Ti precursor solution 12 is connected to a carrier gas supply 10and placed in an ultrasonic humidifier 14. Aerosol droplets 22 arecarried into a reactor tube 20. The reactor tube 20 is positioned in atube furnace 16 with heating zones 18. The aerosol droplets 22 depositon substrate slides 24 within the reactor tube 20. To support a flow ofaerosol, the reactor tube 20 is also connected to a gas trap 26 and anexhaust line 28.

According to a third aspect, the present disclosure relates to anelectrochemical cell, which is made of the composite thin film electrodeof the first aspect, a counter electrode, and an electrolyte solution incontact with both electrodes.

In one embodiment, the electrochemical cell is a vessel having aninternal cavity for holding the electrolyte solution. The vessel may becylindrical, cuboid, frustoconical, spherical, or some other shape. Thevessel walls may comprise a material including, but not limited to,glass, polypropylene, polyvinyl chloride, polyethylene, and/orpolytetrafluoroethylene, and the vessel walls may have a thickness of0.1-3 cm, preferably 0.1-2 cm, more preferably 0.2-1.5 cm. The internalcavity may have a volume of 2 mL-100 mL, preferably 2.5 mL-50 mL, morepreferably 3 mL-20 mL. In another embodiment, for instance, for smallscale or benchtop anodization, the internal cavity may have a volume of100 mL-50 L, preferably 1 L-20 L, more preferably 2 L-10 L. In anotherembodiment, for instance, for pilot plant anodization, the internalcavity may have a volume of 50 L-10,000 L, preferably 70 L-1,000 L, morepreferably 80 L-2,000 L. In another embodiment, for instance, forindustrial plant-scale anodization, the internal cavity may have avolume of 10,000 L-500,000 L, preferably 20,000 L-400,000 L, morepreferably 40,000 L-100,000 L. In one embodiment, one or moreelectrochemical cells may be connected to each other in parallel and/orin series. In another embodiment, the electrolyte solution may be incontact with more than composite thin film electrode and/or more thanone counter electrode.

The counter electrode may comprise platinum, gold, or some other metal.Preferably the counter electrode is a platinum wire. The counterelectrode may also be called an auxiliary electrode. Unless otherwisenoted, the phrase “the electrodes” refers to both the composite thinfilm electrode and the counter electrode.

In one embodiment, the counter electrode comprises gold, platinum, orcarbon. In a further embodiment, the counter electrode comprisesplatinum. In one embodiment, the counter electrode may be in the form ofa wire, a rod, a cylinder, a tube, a scroll, a sheet, a piece of foil, awoven mesh, a perforated sheet, or a brush. The counter electrode may bepolished in order to reduce surface roughness or may be texturized withgrooves, channels, divots, microstructures, or nanostructures.

In another further embodiment, where the counter electrode comprisesplatinum, the counter electrode is in the form of rod or wire.Alternatively, the counter electrode may comprise some otherelectrically-conductive material such as platinum-iridium alloy,iridium, titanium, titanium alloy, stainless steel, gold, cobalt alloyand/or some other electrically-conductive material, where an“electrically-conductive material” as defined here is a substance withan electrical resistivity of at most 10⁻⁶ Ω·m, preferably at most 10⁻⁷Ω·m, more preferably at most 10⁻⁸ Ω·m at a temperature of 20-25° C.

In a preferred embodiment, the counter electrode has at least one outersurface comprising an essentially inert, electrically conductingchemical substance, such as platinum, gold, or carbon. In anotherembodiment, the counter electrode may comprise solid platinum, gold, orcarbon. The form of the counter electrode may be generally relevant onlyin that it needs to supply sufficient current to the electrolytesolution to support the current required for electrochemical reaction ofinterest. The material of the counter electrode should thus besufficiently inert to withstand the chemical conditions in theelectrolyte solution, such as acidic or basic pH values, withoutsubstantially degrading during electrochemical or photoelectrochemicalprocesses. The counter electrode preferably should not leach out anychemical substance that interferes with the electrochemical reaction ormight lead to undesirable contamination of either electrode.

In a further embodiment, where the counter electrode comprises platinum,the counter electrode may be in the form of a mesh. In one embodiment,the counter electrode in the form of a mesh may have a nominal apertureor pore diameter of 0.05-0.6 mm, preferably 0.1-0.5 mm, more preferably0.2-0.4 mm, and/or a wire diameter of 0.01-0.5 mm, preferably 0.08-0.4mm, more preferably 0.1-0.3 mm. In other embodiments, the counterelectrode may be considered a gauze with a mesh number of 40-200,preferably 45-150, more preferably 50-100. In other embodiments, thecounter electrode may be in the form of a perforated sheet or a sponge.In one embodiment, the counter electrode may be in the form of a meshwith one or more bulk dimensions (length, width, or thickness) aspreviously described.

Preferably, the counter electrode is in the form of a rod or wire. Therod or wire may have straight sides and a circular cross-section,similar to a cylinder. A ratio of the length of the rod or wire to itswidth may be 1,500:1-1:1, preferably 500:1-2:1, more preferably300:1-3:1, even more preferably 200:1-4:1. The length of the rod or wiremay be 0.5-50 cm, preferably 1-30 cm, more preferably 3-20 cm, and along wire may be coiled or bent into a shape that allows the entire wireto fit into an electrochemical cell. The diameter of the rod or wire maybe 0.5-20 mm, preferably 0.8-8 mm, more preferably 1-3 mm. In someembodiments, a rod may have an elongated cross-section, similar to aribbon or strip of metal.

Preferably the electrochemical cell also includes a reference electrodein contact with the electrolyte solution. A reference electrode is anelectrode which has a stable and well-known electrode potential. Thehigh stability of the electrode potential is usually reached byemploying a redox system with constant (buffered or saturated)concentrations of each relevant species of the redox reaction. Areference electrode may enable a potentiostat to deliver a stablevoltage to the working electrode or the counter electrode. The referenceelectrode may be a standard hydrogen electrode (SHE), a normal hydrogenelectrode (NIHE), a reversible hydrogen electrode (RHE), a saturatedcalomel electrode (SCE), a copper-copper(II) sulfate electrode (CSE), asilver chloride electrode (Ag/AgCl), a pH-electrode, apalladium-hydrogen electrode, a dynamic hydrogen electrode (DHE), amercury-mercurous sulfate electrode, or some other type of electrode. Ina preferred embodiment, a reference electrode is present and is a silverchloride electrode (Ag/AgCl).

In one embodiment, the electrolyte solution comprises water and aninorganic base The inorganic base may be NaOH, KOH, LiOH, Mg(OH)₂,Ca(OH)₂, Ba(OH)₂, NH₄OH, or some other inorganic base. Preferably theinorganic base is NaOH. In alternative embodiments, an organic base maybe used, such as sodium carbonate or sodium acetate. The inorganic basemay have a concentration of 0.5-1.5 M, preferably 0.75-1.25 M, morepreferably 0.9-1.1 M, or about 1.0 M. In other embodiments, theelectrolyte solution may comprise some other salt, in addition to or inplace of the inorganic base. The salt may comprise at least one cationselected from the group consisting of K⁺, Na⁺, Li⁺, Cu²⁺, Ag⁺, Ni²⁺,Co²⁺, Co³⁺, Zn²⁺, Sn²⁺, Pb²⁺, Fe²⁺, Fe³⁺, Cr²⁺, and Cr³⁺. The counterion of the salt may be SO₄ ²⁻, Br⁻, NO₃ ⁻, OH⁻, Cl⁻, acetate, or someother anion. The salt may have a concentration of 0.02-1.0 M, preferably0.05-0.8 M, more preferably 0.08-0.5 M, or about 0.1 M, though in someembodiments, the concentration may be less than 0.02 M or greater than1.0 M.

In a preferred embodiment, the electrolyte solution may have a totalvolume of 1 mL-10 L, preferably 5 mL-1 L, more preferably 10 mL-500 mL,even more preferably 15 mL-300 mL.

In one embodiment, the composite thin film electrode has a width and/orlength in contact with the electrolyte solution of 0.1-5.0 cm,preferably 0.3-4.0 cm, more preferably 0.5-3.0. The composite thin filmelectrode may have a surface area of 0.5-10 cm² preferably 0.7-5 cm 2,more preferably 1-3 cm² in contact with the electrolyte solution.

In one embodiment, the composite thin film electrode has a currentdensity of 0.45-0.80 mA/cm², preferably 0.50-0.70 mA/cm², morepreferably 0.55-0.65 mA/cm², or about 0.61 mA/cm² when the electrodesare subjected to a bias potential of 0.6-0.8 V and an illumination powerdensity of 80-150 mW/cm², 90-110 mW/cm², preferably 95-105 mW/cm², morepreferably 98-102 mW/cm², or about 100 mW/cm².

The source of the illumination may be a solar simulator that may use axenon arc lamp or a halogen lamp. In one embodiment, the radiationsource may be fitted with a dichroic reflector and/or an optical filterin order to better reproduce solar light illumination, such asillumination having an AM1.5G spectrum, which may be known as the“global standard” spectrum.

As described here, the air mass coefficient (AM) defines the directoptical path length through the Earth's atmosphere, expressed as a ratiorelative to the path length vertically upwards, i.e. at the zenith. Theair mass coefficient can be used to help characterize the solar spectrumafter solar radiation has traveled through the atmosphere. The air masscoefficient is commonly used to characterize the performance of solarcells under standardized conditions, and is often referred to using thesyntax “AM” followed by a number. The air mass number is dependent onthe Sun's elevation path through the sky and therefore varies with timeof day and with the passing seasons of the year, and with the latitudeof the observer. For characterizing terrestrial power-generating panels,the “AM1.5” standard is commonly used for illumination. “AM1.5”represents sunlight through a 1.5 atmosphere thickness, whichcorresponds to a solar zenith angle of 48.2°. The spectrum may besimilar to those defined by ASTM G-173 and IEC 60904 standards. In otherembodiments, the illumination may have an AM1.5D spectrum, which isknown as the “direct standard” spectrum. In alternative embodiments, adifferent illumination standard may be used, such as AM0, AM1, AM2, AM3,or AM38.

In one embodiment, when the composite thin film electrode of theelectrochemical cell is illuminated as above, the composite thin filmelectrode has a charge transfer resistance (R_(ct)) in a range of200-400Ω, preferably 200-400Ω, more preferably 200-400Ω.

According to a fourth aspect, the present disclosure relates to a methodof photocatalytic water splitting. In general, photocatalytic watersplitting is an artificial photosynthesis process with photocatalysis ina photoelectrochemical cell used for the dissociation of water into itsconstituent parts, H₂ and O₂, using either artificial or natural light.Here, this method involves irradiating the electrochemical cell of thethird aspect with sunlight. Alternatively, the electrochemical cell maybe irradiated with light from a gas discharge lamp (such as a mercuryvapor lamp, a xenon lamp, an argon lamp, or a metal halide lamp), alaser, an LED, and/or an incandescent bulb. In one embodiment, a lamp asa solar simulator may be used.

In one embodiment, the solar simulator output to the composite thin filmelectrode may be 40-160 mW/cm², preferably 50-150 mW/cm², morepreferably 90-110 mW/cm², or about 100 mW/cm².

During the irradiating, a bias voltage of 0.5-0.9 V, preferably 0.6-0.8V, or about 0.7 V may be applied to the composite thin film electrodeand the counter electrode. Preferably the composite thin film electrodefunctions as the cathode, receiving a negative potential to reduce waterinto H₂ gas and OH⁻, while the counter electrode functions as the anode,receiving a positive potential to oxidize OH⁻ into O₂ gas and H₂O. Thisis summarized by the following reactions:

2H₂O_((l))+2e⁻→H₂(g)+²OH⁻ _((aq))  Cathode (reduction):

⁴OH⁻ _((aq))→O_(2(g))+2H₂O_((l))+4e⁻  Anode (oxidation):

2H₂O_((l))→2H_(2(g))+O_(2(g))  Overall reaction:

In another embodiment, the potentials may be switched, wherein thecomposite thin film electrode functions as the anode and receives apositive potential, and the counter electrode functions as the cathodeand receives a negative potential. In an alternative embodiment, theelectrodes may be subjected to an alternating current (AC) in which theanode and cathode roles are continually switched between the twoelectrodes.

In one embodiment, the potential may be applied to the electrodes by abattery, such as a battery comprising one or more electrochemical cellsof alkaline, lithium, lithium-ion, nickel-cadmium, nickel metal hydride,zinc-air, silver oxide, and/or carbon-zinc. In another embodiment, thepotential may be applied through a potentiostat or some other source ofdirect current, such as a photovoltaic cell. In one embodiment, apotentiostat may be powered by an AC adaptor, which is plugged into astandard building or home electric utility line. In one embodiment, thepotentiostat may connect with a reference electrode in the electrolytesolution. Preferably the potentiostat is able to supply a relativelystable voltage or potential. For example, in one embodiment, theelectrochemical cell is subjected to a voltage that does not vary bymore than 5%, preferably by no more than 3%, preferably by no more than1.5% of an average value throughout the subjecting. In anotherembodiment, the voltage may be modulated, such as being increased ordecreased linearly, being applied as pulses, or being applied with analternating current. Preferably, the composite thin film electrode maybe considered the working electrode with the counter electrode beingconsidered the auxiliary electrode. However, in some embodiments, thecomposite thin film electrode may be considered the auxiliary electrodewith the counter electrode being considered the working electrode.

In one embodiment, the method further comprises the step of separatelycollecting H₂-enriched gas and O₂-enriched gas. In one embodiment, thespace above each electrode may be confined to a vessel in order toreceive or store the evolved gases from one or both electrodes. Thecollected gas may be further processed, filtered, or compressed.Preferably the H₂-enriched gas is collected above the cathode, and theO₂-enriched gas is collected above the anode. The electrolytic cell, oran attachment, may be shaped so that the headspace above the compositethin film electrode is kept separate from the headspace above thereference electrode. In one embodiment, the H₂-enriched gas and theO₂-enriched gas are not 100 vol % H₂ and 100 vol % O₂, respectively. Forexample, the enriched gases may also comprise N₂ from air, and watervapor and other dissolved gases from the electrolyte solution. TheH₂-enriched gas may also comprise O₂ from air. The H₂-enriched gas maycomprise greater than 20 vol % H₂, preferably greater than 40 vol % H₂,more preferably greater than 60 vol % H₂, even more preferably greaterthan 80 vol % H₂, relative to a total volume of the receptaclecollecting the evolved H₂ gas. The O₂-enriched gas may comprise greaterthan 20 vol % O₂, preferably greater than 40 vol % O₂, more preferablygreater than 60 vol % O₂, even more preferably greater than 80 vol % O₂,relative to a total volume of the receptacle collecting the evolved O₂gas. In some embodiments, the evolved gases may be bubbled into a vesselcomprising water or some other liquid, and higher concentrations of O₂or H₂ may be collected. In one embodiment, evolved O₂ and H₂, orH₂-enriched gas and O₂-enriched gas, may be collected in the samevessel.

Several parameters for the method for decomposing water may be modifiedto lead to different reaction rates, yields, and other outcomes. Theseparameters include, but are not limited to, electrolyte type andconcentration, pH, pressure, solution temperature, current, voltage,stirring rate, electrode surface area, size of CaTiO₃—TiO₂ particles,thickness of the CaTiO₃—TiO₂ layer, and exposure time. A variable DCcurrent may be applied at a fixed voltage, or a fixed DC current may beapplied at a variable voltage. In some instances, AC current or pulsedcurrent may be used. A person having ordinary skill in the art may beable to adjust these and other parameters, to achieve different desirednanostructures. In other embodiments, the electrochemical cell may beused for other electrochemical reactions or analyses.

In an alternative embodiment, the composite thin film electrode may beused in the field of batteries, fuel cells, photochemical cells, watersplitting cells, electronics, water purification, hydrogen sensors,semiconductors (such as field effect transistors), magneticsemiconductors, capacitors, data storage devices, biosensors (such asredox protein sensors), photovoltaics, liquid crystal screens, plasmascreens, touch screens, OLEDs, antistatic deposits, optical coatings,reflective coverings, anti-reflection coatings, and/or reactioncatalysis. Similarly, in one embodiment, the composite thin filmelectrode may be coated with another material. For example, thecomposite thin film electrode may be coated with a layer of gold. Agold-coated composite thin film electrode may then be used for analytedetection using surface enhanced Raman scattering (SERS).

The examples below are intended to further illustrate protocols forpreparing, characterizing the composite thin film electrode, and usesthereof, and are not intended to limit the scope of the claims.

Example 1 Materials and Methods

All the reagents and chemicals were supplied by Sigma-Aldrich. All thechemicals were used as received except tetrahydrofuran, which wasrigorously dried over sodium benzophenoate and distilled immediatelybefore use. All the synthetic work was carried out in Schlenk tubesfitted with the vacuum lines, using hot plates for temperature control,and using dry argon for an inert atmosphere. The controlled thermalanalyses were performed on a Perkin Elmer TGA 4000 thermogravimetricanalyzer equipped with a computer interface. These high temperaturemeasurements were carried out in ceramic crucibles under an inertatmosphere of flowing nitrogen (flow rate=25 mL min-¹) with atemperature ramping of 10° C. min⁻¹. The melting point determinationswere made in a capillary tube using an electrothermal apparatus fromMitamura Riken Kogyo (Japan) Model: MP.D. Fourier transform infra-red(FT-IR) spectra were recorded using a single reflectance ATR instrument(4000-450 cm⁻¹, resolution 4 cm⁻¹). NMR spectra were obtained using aJEOL DELTA2 NMR spectrometer. These experiments were performed at afield strength of 400 MHz in Methanol-D4 as the solvent.

Example 2

Experimental—Synthesis of [Ca₂(TFA)₃(OAc)(^(i)ProH)(H₂O)(THF)₃] (1)

In an effort to synthesize a heterobimetallic Ca—Ti complex, a mixtureof Ca(OAc)₂.2H₂O (0.5 g, 2.63 mmol), Ti(^(i)Pro)₄ (1.55 mL, 5.26 mmol)and trifluoroacetic acid (0.58 mL, 7.90 mmol) in 20 mL THF was stirredfor 2 h in a schlenk tube under an inert atmosphere of dry argon.

The reaction mixture was evaporated under vacuum which resulted in awhite powder. This material was re-dissolved in THF. The transparentsolution obtained was cannula-filtered and placed overnight at roomtemperature to obtain colourless block shaped crystals of complex (1)(75% yield).

Mp: 220° C. (decomposition)¹H NMR (MeOH): δ=3.87 ppm [m, H, CH](^(i)PrOH), δ=3.68 ppm [t, 12H, OCH2] (THF), δ=2.12 [s, 1H, OH](^(i)PrOH), δ=1.12 ppm [d, 6H, CH₃] (^(i)PrOH), δ=1.83 ppm [s, 12H, CH₂](THF), δ=1.96 ppm [s, 3H, CH₃] (OAc), ⁹F NMR (MeOH): δ=−76.94 ppm [s,9F, CF₃]. IR: ν_(max)/cm⁻¹ 3326br, 2983w, 2886w, 1665s, 1452s, 1194s,1141s, 1038s, 884s, 796s, 719s, 652s, 607s, 520w, 463w. TGA: 70-120° C.(3.0% wt. loss); 135-235° C. (28.0% wt. loss); 238-500° C. (51.0% wt.loss), (Residual mass of 18.0%); (Cal. for CaO≈15.0%). Allcharacterization and analysis results are summarized in Table 1.

TABLE 1 Analysis and characterization of complex[Ca₂(TFA)₃(OAc))(^(i)PrOH)(H₂O)(THF)₃] (1) Single crystal ¹H-NMR ¹⁹F-NMRIR TGA analysis (ppm) (ppm) (cm⁻¹) Temp, vs % wt. loss Determination ofchemical δ = 3.87 δ = −76.94 3326 (O—H) 1^(st) step: 70-120° C. formula[m, H, CH] (^(i)PrOH) [s, 9F, CF₃] 2983, 2886 (3.0% wt. loss)C₁₅H₂₁Ca₂F₉O₁₁•2(C₄H₈O) δ = 2.12 (C—H of ^(i)PrOH) 2^(nd) step: 135-235°C. [s, 1H, OH] (^(i)PrOH) 1665, 1452 (28.0% wt. loss) δ = 1.12 (C═O ofCF₃COO⁻) 3^(rd) step: 238-500° C. [d, 6H, CH₃] (^(i)PrOH) 1538, 1370(51.0% wt. loss) δ = 3.68 (C═O of CH₃COO⁻) Residual mass of 18.0% = CaO[t, 12H, OCH₂] (THF) 1194 (C—F) δ = 1.83 ppm 1141 (C—O) [s, 12H, CH₂](THF) δ = 1.96 ppm [s, 3H, CH₃] (OAc)

Example 3 Experimental—Single-Crystal X-Ray Crystallographic Analyses

Single crystal X-ray data was collected at 150(2) K on a SYNERGY,DUALFLEX, ATLASS2 Diffractometer using CuK_(α) radiation (λ=1.54184 Å)and was corrected for the effects of Lorentz-polarization andabsorption. Using ShelXle, the structure of the resulting crystals wassolved and refined by dual space methods (SHELXT) as an inversion twin,on F² using all the reflections (SHELXL-2014). See C. B. Hubschle, G. M.Sheldrick, B. Dittrich, J. Appl. Crystallogr. 44, 1281-1284 (2011); G.M. Sheldrick, Acta Crystallogr. A 71, 3-8 (2015); and G. M. Sheldrick,Acta Crystallogr. C 71, 3-8 (2015), each incorporated herein byreference in their entirety. During the refinement process, allnon-hydrogen atoms were refined by anisotropic atomic displacementparameters whereas the hydrogen atoms were inserted at calculatedpositions using a riding model. The only exception was the hydrogenatoms coordinated with water molecule, which were located fromdifference maps and their coordinates were refined. The CF₃ groups ontwo of the CF₃COO⁻ ions were disordered and modelled using two positionsrelated by rotation; one of the uncoordinated THF molecules was alsomodelled as disordered over two overlapping conformations. All theseparameters of data collection and refinement are briefly described inTable 2. CCDC 1531480 contains the supplementary crystallographic datafor this work. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

TABLE 2 Crystal data and refinement parameters for[Ca₂(TFA)₃(OAc))(^(i)PrOH)(H₂O)(THF)₃] (1) Crystal data Chemical formulaC₁₅H₂₁Ca₂F₉O₁₁•2(C₄H₈O) M_(r) 772.68 Crystal system, space groupOrthorhombic, P2₁2₁2₁ Temperature (K) 150 a, b, c (Å) 13.4102 (3),15.5099 (4), 16.8614 (4) V (Å³) 3507.02 (15) Z 4 Radiation type Cu Kα μ(mm⁻¹) 3.78 Crystal size (mm) 0.20 × 0.18 × 0.11 T_(min), T_(max) 0.512,0.698 No. of measured, independent and 20406, 7209, 6359 observed [I >2σ(I)] reflections R_(int) 0.037 (sin θ/λ)_(max) (Å⁻¹) 0.631 R[F² >2σ(F²)], wR(F²), S 0.050, 0.148, 1.07 No. of reflections 7209 No. ofparameters 531 No. of restraints 1219 Δ > _(max), Δ > _(min) (e Å⁻³) 1.44, −0.38 Absolute structure parameter  0.278 (13)

Example 4 Experimental—Thin Film Fabrication by AACVD

1:1 ratio CaTiO₃:TiO₂ composite oxides films were grown on commerciallyavailable FTO glass substrates having a resistivity of 8 Ωsq⁻¹ usingin-house built setup for aerosol-assisted chemical vapour deposition.The substrates with lateral dimensions 1×2 cm² were ultrasonically andsequentially cleaned with doubly distilled water, acetone andpropan-2-ol, and stored in ethanol until further steps. When ready, thesubstrate slides were removed from ethanol and air dried before beingtransferred to the combustion chamber of AACVD (CAIRBOLITE, Model No.10/25/130) (6″ L×1″ D). The substrates were then heated to the desireddeposition temperature and kept at the same temperature for 10 minutesto get the thermal equilibrium. The AACVD setup is illustrated in FIG. 1.

During a typical deposition experiment, a homogeneous mixture ofCa-precursor [Ca₂(TFA)₃(OAc)(^(i)PrOH)(H₂O)(THF)₃] (0.5 g, 0.65 mmol)and titanium (IV) isopropoxide (0.06 ml, 0.22 mmol) was prepared in 15mL methanol as solvent. This mixture was utilized for the fabrication ofCaTiO₃—TiO₂ composite thin films at three different temperatures of 500,550, and 600° C. The deposition experiments were continued for a fixedtime period of 60 min. at each temperature. Thin film depositionparameters are listed in Table 3. Visual examination indicated thatfilms were white, uniform, and adhered strongly to FTO substrate asconfirmed by the “SCOTCH tape test”.

TABLE 3 Thin films deposition parameters involved in AACVD synthesis ofCaTiO₃—TiO₂ composite film. Concentration Deposition of precursorstemperature Ca-Complex + Solvent Deposition Carrier Resultant (° C.)Ti(^(i)Pro)₄ (mL) time (min.) Substrate Gas/(mL/min.) Material 500 (0.5g, 0.65 MeOH 60 FTO 150 CaTiO₃—TiO₂ mmol) + (0.06 (15) ml, 0.22 mmol)550 (0.5 g, 0.65 MeOH 60 FTO 150 CaTiO₃—TiO₂ mmol) + (0.06 (15) ml, 0.22mmol) 600 (0.5 g, 0.65 MeOH 60 FTO 150 CaTiO₃—TiO₂ mmol) + (0.06 (15)ml, 0.22 mmol)

Example 5 Experimental—Thin Film Characterization

The UV-Vis absorption spectra of the thin films were recorded on aLAMBDA 35 Perkin-Elmer spectrophotometer in the wavelength range of300-900 nm. Film thicknesses were measured by using a KLA TENCORE P-6surface profiler. In order to verify the crystallinity of the formedcomposite oxide, the films were examined by X-ray powder diffraction(XRD) using a PANanalytical, X'Pert HighScore diffractometer withprimary monochromatic high intensity CuK_(α)(λ=1.5418 Å) radiation.Energy dispersive X-ray (EDX, INCA Energy 200, Oxford Inst.)spectroscopy was also carried out in order to determine the metallicratio of Ca/Ti in the composite. The microstructure of the thin filmswere investigated using a Lyra 3 Tescan, field emission gun (FEG)-SEM atan accelerating voltage of 5 kV and a working distance of 10 mm. X-rayphotoelectron spectroscopy (XPS) was undertaken using an ULVAC-PHIQuantera II instrument with a 32-channel Spherical Capacitor EnergyAnalyzer under vacuum pressure of 1×10⁻⁶ Pa using Monochromated Al Kαradiation (1486.8 eV) and natural energy width of 680 meV. Thecarbonaceous C is line (284.6 eV) was used as a reference to calibratethe binding energies.

Example 6 Experimental—Photoelectrochemical Measurements

The photoelectrochemical responses of the resulting CaTiO₃—TiO₂composite electrodes were measured using a conventional three-electrodecell configuration on a source of external potential bias (PrincetonApplied Research PAR-VersaSTAT-3). Current-voltage characteristics wereobserved using the linear sweep voltammetry (LSV) technique for apotential range of −0.5 to +1.2 V at a scan rate of 10 mV s⁻¹. TheCaTiO₃—TiO₂ thin films were used as the working electrode in theseexperiments while having platinum wire as a counter electrode and aAg/AgCl reference electrode. The working electrodes (i.e., CaTiO₃—TiO₂thin films) were dipped into 1 M NaOH electrolyte solutions for thecalculations of the photocurrent, and irradiated with a 100 mW xenonlamp (Newport, Model 69907) containing a simulated AM 1.5G filter as alight source, during the entire length of the experiment.

Example 7

Results and Discussion—an Insight into Complex (1)

A polymeric complex of calcium [Ca₂(TFA)₃(OAc))(^(i)PrOH)(H₂O)(THF)₃](1) (TFA=trifluoroacetato; OAc=acetato; ^(i)Pro=isopropoxy) has beenobtained as a result of efforts to synthesize a heterobimetallic Ca—Ticomplex. The reactants Ca(OAc)₂.2H₂O and Ti(^(i)Pro)₄ were mixed in themolar ratio of 1:2 in tetrahydrofuran solvent followed by subsequentaddition of triflouroacetic acid with the aim of achieving a Ca—Tibimetallic compound to be implemented in AACVD for the fabrication ofCaTiO₃—TiO₂ composite oxide photelectrodes for PEC water splitting. Thesynthetic pathway did not successfully produce our desired Ca—Tibimetallic complex, but instead produced complex (1) which was stillinteresting with regard to AACVD application. Hence we utilized ahomogenous methanolic solution of complex (1) and Ti(^(i)Pro)₄ tofabricate CaTiO₃—TiO₂ oxide thin film electrodes. The complex (1) wasproduced in the form of transparent white crystals having a 75% yield.The obtained crystals were quite stable towards air and moisture whileexhibiting appreciable solubility in routinely used organic solventssuch as methanol, ethanol, acetonitrile, etc. The structural compositionof the formed complex (1) was initially established by single crystalXRD and was further ascertained by AT-IR, ¹H and ¹⁹F NMR, andthermogravimetry analyses (TGA). The IR spectra of complex (1) indicatethat different functional moieties are attached to the calcium centersin (1). At 3326 cm⁻¹, a broad peak corresponding to OH functional groupis present, and at 2983 and 2886 cm⁻¹, there are peaks to indicate theCH group of isopropanol. See X. Cai, Y. Wu, L. Wang, N. Yan, J. Liu, X.Fang, Y. Fang, Soft Matter 9, 5807-5814 (2013), incorporated herein byreference in its entirety. The symmetric and asymmetric υ(C═O)absorption signals of the acetato and trifluoroacetato ligands have alsobeen observed at 1538 and 1370 cm⁻¹, and 1665 and 1452 cm⁻¹,respectively. The difference of 168 and 213 cm⁻¹ between these symmetricand asymmetric υ(C═O) vibrations reveals the bidentate characteristicsof the carboxylate groups of the acetato and trifluoroacetato groupsthat are coordinated to different calcium atoms. See M. Veith, M. Haasand V. Huch, Chem. Mater. 17, 95-101 (2005), incorporated herein byreference in its entirety. In addition, the peaks at 1194 and 1141 cm⁻¹authenticate the occurrence of C—F and C—O bonds in complex (1). ¹H-NMRspectra measured from D-methanol solution displays three signals at,δ=3.87 ppm [p, 4H, CH] δ=2.12 ppm [s, 1H, OH] and 1.12 ppm [d, 6H, CH₃]due to the isopropanol, while two signals at δ=3.68 ppm [t, 12H, OCH₂]and δ=1.83 ppm [t, 12H, CH₂] are for the solvated THF and δ=1.96 ppm [s,3H, CH₃] are for the acetato groups. Further, the ¹⁹F-NMR spectrarecorded in methanol displays a singlet at δ=−76.94 ppm [s, 9F, CF₃]suggesting the presence of a trifluoroacetato moiety in the formedcomplex (1).

Example 8 Results and Discussion—Description of Molecular Structure ofComplex (1)

The asymmetric unit contains two independent calcium ions linked bybridging acetate and trifluoroacetate groups as shown in FIG. 2 , whereH atoms and non-coordinated THF molecules are omitted; hydrogen bondingis shown as black dashed lines. Ca²⁺ and Ti⁴⁺ are isoelectronic, socannot be readily distinguished only on the basis of X-ray data.However, the M-O bond length data with related assemblies in the CSD(v5.37 & updates) support assignment as Ca²⁺ and are inconsistent withTi⁴⁺. See F. H. Allen, Acta Crystallogr. B 58, 380-388 (2002),incorporated herein by reference in its entirety. Furthermore, replacingeven one of the Ca²⁺ ions with Ti⁴⁺ would present difficulties withcharge balance, since there is only one ionisable proton remaining inthe asymmetric unit (on the isopropanol).

The two calcium ions are linked on one side by two syn bridgingbidentate CF₃COO ions and by a single atom bridge from the acetateligand; on the other by one syn bidentate CF₃COO ion and a single oxygendonor from the acetate ligand. The acetate ligand acts as a synbidentate ligand to Ca2 and as an anti bidentate bridge linking Ca1 to asymmetry equivalent atom (under synmmetry operation x−1/2, −y+3/2,−z+1). Ca1 is 6-coordinate, the coordination sphere completed by anisopropanol ligand while Ca2 is 7-coordinate, having water and THEligands. Between the coordinated alcohol on Ca1 and the water moleculeon Ca2 (under x−1/2, −y+3/2, −z+1), an intramolecular hydrogen bond islocated whereas two hydrogen bonds are directed from the coordinatedwater molecule, one to each of the uncoordinated THF molecules (Table4).

TABLE 4 Hydrogen-bond geometry (Å, °) for[Ca₂(TFA)₃(OAc))(^(i)Pro)(H₂O)(THF)₃] D-H . . . A D-H H . . . A D . . .A D-H . . . A O1—H2 . . . O71 0.82 (8) 1.88 (8) 2.683 (6) 169 (8) O1—H1. . . O81 0.86 (8) 1.92 (8)  2.718 (14) 155 (7) O1—H1 . . . O81′ 0.86(8) 1.81 (8)  2.628 (13) 159 (7) O51—H5LA . . . O1^(i) 0.84 2.07 2.902(6) 179 Symmetry code: (i) x − 1/2, −y + 3/2, −z + 1.

Example 9 Results and Discussion—Pyrolysis of Complex (1)

Simultaneous thermogravimetric analysis (TGA) and derivativethermogravimetric (DTG) analyses were performed under a flowingdinitrogen atmosphere (flow rate=25 cm³ min⁻¹) and with a temperatureramping of 10° C. min⁻¹ in order to investigate the thermal stability ofcomplex (1). The thermogram thus obtained (FIG. 3 ) illustrates that thedecomposition of (1) proceeds into three discrete stages with acontinuous weight loss of 3.0%, 28.0% and 51.0%, with the maximum heatintake steps at 98° C., 220° C. and 272° C., respectively. At 600° C.,the amount of the residue obtained is ˜18.0%, which suggest theformation of “n” moles of CaO (calculated percentage ˜15.0%). Furtherheating of the residue up to 800° C. brought no significant change inmass, indicating complete decomposition of complex (1) yielding a stableresidue of CaO. Quantitative pyrolysis of complex (1) has been indicatedin the equation below, based on TGA information:

EXAMPLE 10 Results and Discussion—XRD Analysis

FIG. 4A shows the XRD patterns of the films of CaTiO₃—TiO₂ compositeformed at three different temperatures. The qualitative phase analysisvia XRD indicates the formation of “perovskite CaTiO₃” ICSD[98-003-7263] and anatase TiO₂ (ICSD=98-000-9853) phases in all thecomposite oxide films. The diffraction peaks indicated by (X) at2θ=23.2°, 32.9°, 33.0°, 33.3° 39.0°, 39.1°, 40.6°, 41.0°, 47.4° 47.5°58.8°, 59.0°, 59.2° 59.3°, 68.9°, 69.4°, 79.0°, 79.2°, 88.0° and 88.9°correspond to the (101), (200), (121), (002), (031), (112), (220),(022), (202), (040), (321), (240), (042), (123), (400), (242), (323),(161), (440), (044) planes, respectively, in good agreement with theorthorhombic CaTiO₃ (FIG. 4A). The peaks labelled as (Y) at 2θ=25.2°(011), 36.9° (013), 37.7° (004), 38.5° (112), 48.0° (020), 53.8° (015),55.0° (121), 62.6° (024), 68.6° (116), 70.2° (220), 75.0° (125) and82.5° (224) are regarded as the attributive indicator of TiO₂ in thetetragonal anatase phase (ICSD=98-000-9853). The Peaks indicated by (*)are diffracted from crystalline tin oxide (ICSD=01-077-2296) of the FTOsubstrate. The XRD patterns do not show any sort of crystalline impuritypeak such as from CaO or any other type of TiO₂.

Although all the films demonstrated similar XRD patterns and thus had asimilar phase of perovskite CaTiO₃ and anatase TiO₂ in crystallinedeposit in each case, however, it is perceived from these XRDs that thedegree of crystallinity of CaTiO₃ and TiO₂ phases are influenced by thetemperature during the deposition. For example, the film deposited at500° C., TiO₂ peak situated at 25.2° appears as highest intensity peakwhile the characteristic CaTiO₃ peaks centered at 47.4° and 47.5° remainlow and various other peaks are absent. With rise in the depositiontemperature the intensity of TiO₂ peak (25.2°) is reduced and CaTiO₃peaks become sharp and various other peaks are quite visible in the filmspecimen developed at 600° C.

To confirm this observation, quantitative phase analysis was applied oneach XRD pattern and crystalline proportions of perovskite CaTiO₃ andanatase TiO₂ phases were measured in each film and are shown in the formof pie-charts (FIGS. 4B-4D).

The XRD quantitative phase analysis reveals that the film deposited at500° C. contains higher percentage of crystalline anatase TiO₂ (73%) ascompared to perovskite CaTiO₃ (27%). The crystalline contents of TiO₂are observed to decrease with increase in deposition temperature andCaTiO₃—TiO₂ composite prepared at 550° C. is poised at 43% of anataseTiO₂ and 57% of perovskite CaTiO₃. The crystalline content of anataseTiO₂ further reduces to 17% and perovskite CaTiO₃ with 83% becomes asmajor phase in the crystalline CaTiO₃—TiO₂ product obtained at 600° C.Worth mentioning here is that the films produced at three differenttemperatures differ in terms of their percentage crystalline compositionof the two phases, however, the total crystalline plus non-crystallinephases of CaTiO₃: TiO₂ remain at 1:1 ratio and can be furtherascertained from elemental (EDX and XPS) analyses of the films. Thevariation in crystalline contents of the films as a result of change indeposition temperature is a common aspect of AACVD where the reactionsbetween precursor gaseous intermediates and substrate surface areprofoundly influenced by the deposition temperature and thus lead to theformation of oxide products with different level crystalline phases. SeeM. A. Ehsan, H. Khaledi, A. Pandikumar, P. Rameshkumar, N. M. Huang, Z.Arifin, M. Mazhar, New J. Chem. 39, 7442-7452 (2015); and A. A. Tahir,M. A. Ehsan, M. Mazhar, K. U. Wijayantha, M. Zeller, A. Hunter, Chem.Mater. 22, 5084-5092 (2010), each incorporated herein by reference intheir entirety.

Example 11 Results and Discussion—SEM/EDX Analysis

The surface and cross sectional FESEM images of CaTiO₃—TiO₂ compositeoxide films developed at 500, 550 and 600° C. are presented in FIG. 4 .The low resolution SEM images (FIGS. 5A, 6A, 7A) reveal that CaTiO₃—TiO₂crystallites are homogeneously distributed on the substrate surface overall temperature ranges. The high resolution SEM images (FIGS. 5B, 6B,7B) unveil the spherical shape of the CaTiO₃—TiO₂ crystallites which areregularly connected with each other and it has been observed that size,shape and texture of these spheroids are not significantly altered withincrease in deposition temperature from 500 to 600° C. The averagediameter of the CaTiO₃—TiO₂ particles have been measured in the sizerange of 0.3-1.8 μm.

However, SEM cross sectional views, FIGS. 5C, 6C, 7C, depict thevariation in film thickness as a function of varying substratetemperature. The CaTiO₃—TiO₂ film developed at 500° C. displayed a layerof spheroid particles of thickness around 3.0 μm which almost becomedouble (˜6.0 μm) when temperature increased to 550° C., suggesting thatfilm growth rates become better with rise in the deposition temperature.The nucleation and growth of spherical particles further enhances withincreasing deposition temperature up to 600° C. A layer of spheroidobjects can be clearly seen on the surface with an average thickness of9.0 μm.

Energy dispersive X-ray analysis (EDX) is used to confirm the elementalcomposition and purity of all the composite films. The EDX spectra(FIGS. 8A-8H, 9A-9H, and 10A-10H) demonstrate the atomic ratio of Ca toTi is about 1:2, confirming the formation of 1:1 for CaTiO₃: TiO₂composite material. EDX patterns also indicate a uniform concentrationof Ca and Ti in all regions examined, indicating that CaTiO₃ and TiO₂phases are uniformly distributed. The various signals resulted fromsubstrate elements (i.e., Sn, Si, Na, F) and Au-coating are not excludedfrom EDX spectra. Further, the composite nature of CaTiO₃—TiO₂ filmswere established from EDX mapping (FIGS. 11A-11D, 12A-12D, 13A-13D)indicating that calcium, titanium, and oxygen atoms are evenlydistributed throughout the films matrices, which confirms the compositenature of all films.

Example 12

Results and Discussion—XPS analysis

In order to further confirm the elemental composition and the chemicalstate of CaTiO₃—TiO₂ composite, XPS measurements were carried out on thefilm fabricated at 600° C. and XPS spectra are illustrated in FIGS.14A-14D. The survey spectrum, (FIG. 14A), reveals that Ca, Ti, and O aremajor elements present on the film surface and Ca/Ti atomic ratio wasfound to be ˜1:2 that is consistent with EDX results. The highresolution XPS spectrum of Ca (FIG. 14B) displays two peaks located atabout 346.0 eV and 351.0 eV corresponding to the Ca2p3/2 and Ca2p1/2,respectively and represents the chemical state of Ca²⁺. See D. Wei, Y.Zhou, D. Jia and Y. Wang, J. Biomed. Mater. Res. B Appl. Biomater. 84,444-451 (2008), incorporated herein by reference in its entirety. Thedouble peak can be de-convoluted from the curve fitting into four subpeaks at 346.3, 347.9, 349.9 and 351.4 eV, FIG. 14B. These resultssuggest the presence of CaTiO₃ (347.9 eV) with some CaCO₃ (349.9 eV) andthese data are in accordance with reported data for CaTiO₃. See S.-W.Lee, L. Lozano-Sánchez, V. Rodriguez-González, J. Hazard. Mater. 263,20-27 (2013), incorporated herein by reference in its entirety. The XPSspectrum of Ti (FIG. 14C) shows two main peaks at 458.0 eV and 464.0 eVthat can be referred to the Ti 2p3/2 and Ti 2pi/2, respectively,confirming the presence of Ti⁴⁺ in CaTiO₃—TiO₂ composite oxides. See D.Boukhvalov, D. Korotin, A. Efremov, E. Kurmaev, C. Borchers, I. Zhidkov,D. Gunderov, R. Valiev, N. Gavrilov, S. Cholakh, Phys. Status Solidi B252, 748-754 (2015), incorporated herein by reference in its entirety.In FIG. 14D, the O 1s profile can be fitted to two symmetrical peaks at529.2 and 529.8 eV, accredited to Ca—O and Ti—O, respectively. The smallpeaks at 531.1 eV can be assigned to the chemisorbed oxygen caused bythe surface hydroxyl (OH). See X-J. Huang et al. (2016).

Example 13 Results and Discussion—Optical Properties

The optical bandgap (E_(g)) of CaTiO₃—TiO₂ composite films grown at 600°C. was investigated by UV-Visible absorption spectrophotometry.UV-visible absorption spectrum (FIG. 15A) of CaTiO₃—TiO₂ display a steepabsorption in the wavelength region of 300-470 nm. The optical band gapof the composite films has been graphically determined from Tauc'sformula valid for direct bandgap semiconductors (i.e.,αhν=A(hν−E_(g))^(1/2)) where A is a constant, α is the absorptioncoefficient, hν is the photon energy, and E_(g) is the band gap energy).

FIG. 15B shows the corresponding Tauc's plot between (αhν)² versusE_(g). A direct band gap value for composite oxide film is derived as3.0 eV from the extrapolated straight lines to the X axis of E_(g) atα=0. The bandgaps for perovskite CaTiO₃ and anatase TiO₂ are reported tobe 3.5 eV and 3.2 eV respectively. See H. Mizoguchi, K. Ueda, M. Orita,S.-C. Moon, K. Kajihara, M. Hirano, H. Hosono, Mater. Res. Bull. 37,2401-2406 (2002); and D. O. Scanlon, C. W. Dunnill, J. Buckeridge, S. A.Shevlin, A. J. Logsdail, S. M. Woodley, C. R. A. Catlow, M. J. Powell,R. G. Palgrave, I. P. Parkin, Nat. Mater. 12, 798-801 (2013), eachincorporated herein by reference in their entirety. The opticalmeasurements of CaTiO₃—TiO₂ composite film show that incorporation ofTiO₂ into perovskite CaTiO₃ leads to a red shift in the optical responseand a concomitant reduction of 0.5 eV in E_(g) of CaTiO₃. It is wellestablished that coupling of two semiconductors results in contractionof optical bandgap due to the synergic effect of the two componentspresent in the composite. See F. Hu and W. Chen, Electrochem. Commun.13, 955-958 (2011); and M. A. Mansoor, M. Mazhar, M. Ebadi, H. N. Ming,M. A. M. Teridi, L. K. Mun, New J. Chem. 40, 5177-5184 (2016), eachincorporated herein by reference in their entirety. The positive effectof synergic cooperation is commonly explained by the opinion that, inthe new equilibrium Fermi energy level of the CaTiO₃—TiO₂ composite, thephoto excited electron on the conduction band of CaTiO₃ are readilytransferred into the TiO₂ particles. The accumulation of excesselectrons in TiO₂ diminishes the recombination of photo-inducedelectrons and holes and thus, improves the photocatalytic activity ofthe films. Similar synergistic effects of TiO₂ with SrTiO₃ and BaTiO₃have been previously reported for their enhanced photocatalyticefficiencies. See S. E. Stanca, R. Müller, M. Urban, A. Csaki, F.Froehlich, C. Krafft, J. Popp, W. Fritzsche, Catal. Sci. Technol. 2,1472-1479 (2012); and J. Ng, S. Xu, X. Zhang, H. Y. Yang and D. D. Sun,Adv. Funct. Mater. 20, 4287-4294 (2010), each incorporated herein byreference in their entirety.

Example 13 Results and Discussion—Photoelectrochemical Performance

To evaluate the photoelectrochemical (PEC) behavior of the CaTiO₃—TiO₂(CT) composite, the films were employed as photoanodes in a 3-arm cell.A Pt counter electrode with 1 M NaOH as an electrolyte were employed,and all CaTiO₃—TiO₂ electrodes (CT-500, CT-550, and CT-600) wereilluminated with simulated sunlight, (100 mW xenon lamp equipped with AM1.5 filters). The current density-voltage (J-V) characteristics ofCaTiO₃—TiO₂ electrodes as recorded in dark (D) and light (L) conditionsare shown in FIG. 16A. The dark current coming from each electrode at−0.4 V to +0.7 V (vs. Ag/AgCl) was almost negligible. A promptphotocurrent response in all cases is observed after the illumination,however, the magnitude of photocurrent varied depending upon thedeposition temperature. Notably, the CT-600 electrode deposited at 600°C. exhibited highest photocurrent of 610 μA cm⁻² at 0.7 V compared tothe CT-550 and CT-500 electrodes, which produced photocurrent densitiesof 530 and 370 μA cm⁻², respectively. This reflects that depositiontemperature is a significant factor influencing the photocurrent densityof the film electrode which ultimately affects the structural propertiesof the CaTiO₃—TiO₂ electrodes such as crystallinity, surface morphology,and thickness. It is evident from XRD and SEM (surface andcross-sectional) results that CT-600 electrode exhibits highercrystallinity and a denser layer (˜9 μm) of microspherical particles(FIG. 7C) than the other two electrodes (CT-550 and CT-500) which arerelatively less crystalline and thinner (˜6 μm and ˜3 μm, respectively).The improved crystalline pattern of CT 600 eliminates the distortion andpoor lattice mismatching between particle-particle orparticle-conducting layer connection and develops better linkage betweenCaTiO₃—TiO₂ crystallites that lead to a reduction in the recombinationand improved charge transport properties in the electrode, thusexhibiting better PEC performance. It is well known that the electrodefabrication method has a strong effect on the crystalline structure andmorphology of the composite film system and thus the performance of thePEC cells. See W. Wang, M. O. Tadé, Z. Shao, Chem. Soc. Rev. 44,5371-5408 (2015), incorporated herein by reference in its entirety.

To further confirm that photoresponses of CaTiO₃—TiO₂ electrodes are dueto the absorption of light, choronoamperometric 1-t studies were carriedout under the white light illumination of 100 mW/cm² with an on-offillumination cycle of 200 s and a bias potential of +0.7 V (vs.Ag/AgCl)). FIG. 16B illustrates that all CaTiO₃—TiO₂ photoanodes wereable to sustain and replicate their photocurrents during long termcontinuous on-off illumination for 1 h without any noticeabledeterioration in photocurrent which reveals their excellent stabilitiesduring the photo-induced electrochemical process. Additionally, thetransient photocurrent values recoded for CT-500 (372 μA cm⁻²), CT 550(527 μA cm⁻²) and CT-600 (610 μA cm⁻²) correspond well with their linearsweep voltametry results, thus strengthening the photoelectrocatalyticperformance of the CaTiO₃—TiO₂ electrodes.

Electrochemical impedance spectroscopy (EIS) was also performed to probethe resistivity of the electrode materials and the interfacialproperties between the electrode and the electrolyte over a frequencyfrom 10 kHz to 0.1 Hz in the presence of 0.1M NaOH. FIG. 17A shows theEIS Nyquist plot of CaTiO₃—TiO₂ electrodes measured in dark andillumination conditions. In the Nyquist plot, the arc of a semicircle athigh frequency range represents the typical charge-transfer process, andthe diameter of the semicircle reflects the charge-transfer resistance(R_(ct)).

As shown in FIG. 17A, the diameter of all CaTiO₃—TiO₂ electrodes wasmuch larger in the dark offering higher R_(ct) which inhibits the chargetransfer process across the electrode-electrolyte interface. However inpresence of light, the Nyquist curves of all CaTiO₃—TiO₂ electrodes areconsiderably reduced in size, indicating lowering of R_(ct) thatfacilitate the fast mobility of electron by reducing the recombinationof electron-hole pairs. The R_(ct) values measured from Nyquist plotsare listed in Table 3. Moreover, the Table 5 compares the R_(ct) valuesof three CT (L) electrodes and reveals that influence of illumination onCT-600 electrode is more pronounced and results in lowest R_(ct) valuerepresenting the efficient rather enhanced charge separation andtransfer across the interface. Moreover, it reduces the possibility ofcharge recombination at the surface of the CT-600 electrode.

Consequently, the EIS findings are completely in agreement withvoltammetry results and further endorse the PEC capabilities ofCaTiO₃—TiO₂ electrodes.

TABLE 5 Charge transfer resistance, maximum frequency and recombinationlifetime calculated for CaTiO₃—TiO₂ film electrodes fabricated atdifferent indicated temperatures via AACVD. Film electrode R_(ct) (ohm)f_(max) (Hz) τ_(n) (msec) CT 500 (D) 2208 265.4 0.59 CT 500 (L) 442 16.29.81 CT 550 (D) 2140 157.6 1.00 CT 550 (L) 442 1.96 81.1 CT 600 (D) 2078137.9 1.11 CT 600 (L) 295 0.5 318 R_(ct)—Charge transfer resistance;f_(max)—Maximum frequency; τ_(n)—recombination lifetime.

The charge-transfer diffusion properties of CaTiO₃—TiO₂ electrodes canbe further established by recording the frequency dependent phase angleplots (Bode plot) in dark and light conditions (FIG. 17B). In presenceof light, the characteristic frequency peak (f_(max)) of all CaTiO₃—TiO₂films are observed to shift towards low frequency region (0.1 Hz) (Table3), suggesting the fast and facile electron-transfer behavior of all CTelectrodes. Further, the low resistance of CT-600 under light exhibitspeak shift more towards the low frequency region in the Bode plot (Table3, FIG. 17B). Under illumination, the phase angle of the plot at higherfrequency is less than 90° and there is lesser log z value in lowfrequency range of 1-100 Hz. This suggests that the electrode does notbehave as an ideal capacitor.

The f_(max) observed for each CaTiO₃—TiO₂ electrode can be used todetermine the electron recombination lifetime (τ_(n)) using thefollowing equation:

τ_(n)=½πf _(max)

This defines the lifetime of charge carrier inside the bulk of theelectrode material and higher τ_(n) value corresponds to the longerlifespan of the charge carrier. The calculated τ_(n) values for allCaTiO₃—TiO₂ electrodes in absence and presence of light are shown inTable 3. The data depicts that the lifetime of charge carrierscorresponding to CaTiO₃—TiO₂ electrodes is prolonged effectively underlight compared to dark conditions. The τ_(n) value is highest ascalculated for CT-600 (L) which in turn confirms the production ofhighest current density in it. Thus, the CaTiO₃—TiO₂ electrode made at600° C. shows smaller interface impedance, higher photocurrent density,and better charge transportation properties as compared to theelectrodes prepared at 500 and 550° C.

The PEC results of CaTiO₃—TiO₂ composite oxide have been compared withother calcium titanate based materials. The photocurrent density of 610μA cm⁻² and electrochemical stability of 1 hour observed in the currentstudies is several time higher than the pristine CaTiO₃ (1.2 μAcm⁻²) andZr-doped CaTiO₃ (100 μAcm⁻²) electrodes. See X. Yan, X. Huang, Y. Fang,Y. Min, Z. Wu, W. Li, J. Yuan, L. Tan, Int. J. Electrochem. Sci. 9,5155-5163 (2014); and X.-j. Huang et al. (2016), each incorporatedherein by reference in their entirety. The other Ag—CaTiO₃ andCaTiO₃-graphene composites were investigated for photocatalysis oforganic molecules and are not relevant to the present work. See S.-W.Lee et al. (2013), and T. Xian et al. (2014). The higher photo-responseof CaTiO₃—TiO₂ composite is attributed to better separation andtransportation of photo-induced holes and electrons at the junction andthis mechanism is depicted in FIG. 18 . The bandgaps of anatase TiO₂(3.2 eV) and perovskite CaTiO₃ (3.5 eV) are aligned in such manner thatincident UV light can simultaneously activate both semiconductormaterials. Upon illumination, electrons from VBs of CaTiO₃ and TiO₂ areexcited and successively promoted to their respective CBs, leavingbehind holes. These holes then perform oxidation atsemiconductor/electrolyte junction. See M. Veith, M. Haas and V. Huch,Chem. Mater. 17, 95-101 (2005), incorporated herein by reference in itsentirety. Since the CB of TiO₂ is ˜0.3V more positive, the promotedelectrons from the CB of CaTiO₃ are shifted there. See S.-W. Lee et al.(2013). The accumulation of excess electrons in TiO₂ causes a negativeshift in its Fermi levels. In a previous photocatalytic investigationbased on titanium-oxide-based composite of perovskite like structure, itwas argued that the electrons flow to the CB of TiO₂ through apn-junction. A pn-junction is an interface where the p-type acceptorcompound (CaTiO₃) and n-type donor compound (TiO₂) connectelectronically. See M. Ueda, S. Otsuka-Yao-Matsuo Sci. Technol. Adv.Mater. 5, 187-193 (2004); and T. Omata, S. Otsuka-Yao-Matsuo, J.Photochem. Photobiol. 156, 243-248 (2003), each incorporated herein byreference in their entirety. The charge flow through the pn-junction mayform highly reduced states of CaTiO₃—TiO₂ that are stable even underoxygen saturated conditions by inducing an efficient spatial separationof the photogenerated interparticle charges. This in turn improves thephotocatalytic water splitting performance. See M. Ueda et al. (2004);and Z. Jin, X. Zhang, Y. Li, S. Li, G. Lu, Catal. Commun. 8, 1267-1273(2007), each incorporated herein by reference in their entirety. TheFermi levels of the undoped, intrinsic n-type TiO₂ semiconductor lies atthe minimum of its CB range. The coupling of CaTiO₃ elevates Fermienergy levels to assume a more negative redox potential compared toH+/H₂ (0 V vs. NHE). Thus an overall higher work function is shown forwater reduction reactions. See Z. Jin et al. (2007); and A. Kudo, Y.Miseki, Chem. Soc. Rev. 38, 253-278 (2009), each incorporated herein byreference in their entirety. The negative shift in Fermi level isindicative of a large accumulation of electrons at the heterojunctionsof TiO₂ and CaTiO₃, reflecting a decrease in recombination of charges.See J. Zhang, J. H. Bang, C. Tang, P. V. Kamat, ACS Nano 4, 387-395(2009), incorporated herein by reference in its entirety. For efficientwater splitting to produce H₂, the CB level should be sufficientlyhigher (more negative) than the reduction potential of H₂O. Thetheoretical minimum bandgap for water splitting has been calculated tobe 1.23 eV. See A. Kudo, H. Kato and I. Tsuji, Chem. Lett. 33, 1534-1539(2004), incorporated herein by reference in its entirety. It is expectedthat the CaTiO₃—TiO₂ composite can optimize light harvesting abilitieswith the incorporation of anatase and perovskite crystallites.

In the present work, photoelectrochemical water splitting has beendemonstrated over robust and stable CaTiO₃—TiO₂ composite electrodeswhich have been developed through solution processed AACVD approachutilizing a mixture of [Ca₂(TFA)₃(OAc))(^(i)PrOH)(H₂O)(THF)₃] andTi((CH₃)₂CHO)₄ as dual source precursors. It has been found that theincorporation of TiO₂ into perovskite CaTiO₃ leads to decrease in theoptical bandgap from 3.5 to 3.0 eV while reducing the possibility ofe⁻-h⁺ recombination, thereby enhancing photocatalytic activity. The PECexperiments on CaTiO₃—TiO₂ electrodes at 0.7 V vs. Ag/AgCl/3M KCl undersimulated solar irradiation of AM 1.5G (100 mW/cm²) show maximum anodicphotocurrent density of 610 μA/cm² in 1M NaOH. This enhanced activity ofthe composite film fabricated at 600° C. is further supported by EISresults that show a maximum frequency peak below 0.1 Hz. These resultsdemonstrate efficient photocatalytic cleavage of water using novelCaTiO₃—TiO₂ composite films prepared through CVD methods. Such uniqueand efficient composite materials may find application in solar energyharvesting via photoelectrochemical cells.

1. (canceled)
 2. The method of claim 8, wherein the crystallineCaTiO₃—TiO₂ composite layer comprises 80-85 wt % CaTiO₃ and 15-20 wt %TiO₂, each relative to a total weight of the crystalline CaTiO₃—TiO₂composite layer.
 3. The method of claim 8, wherein the crystallineCaTiO₃—TiO₂ particles of the crystalline CaTiO₃—TiO₂ composite layer aresubstantially spherical.
 4. The method of claim 8, wherein the TiO₂ ofthe crystalline CaTiO₃—TiO₂ composite layer is in anatase phase.
 5. Themethod of claim 8, wherein the CaTiO composite thin film electrode has adirect band gap value in a range of 2.5-3.5 eV.
 6. The method of claim8, wherein the conductive transparent substrate is a transparentconducting film selected from the group consisting of ITO, FTO, AZO,GZO, IZO, IZTO, IAZO, IGZO, IGTO, and ATO.
 7. The method of claim 8,wherein the conductive transparent substrate has a sheet resistance in arange of 1-40 Ωsq⁻¹.
 8. A method of making a CaTiO composite thin filmelectrode for water splitting, the method comprising: contacting anaerosol with a conductive transparent substrate to deposit a crystallineCaTiO₃—TiO₂ composite layer on the conductive transparent substrate toform the CaTiO composite thin film electrode, wherein the aerosolcomprises a carrier gas and a calcium complex and a titanium complexdissolved in a solvent, and wherein the conductive transparent substratehas a temperature in a range of 400-650° C. during the contacting,wherein the CaTiO composite thin film electrode, comprises thecrystalline CaTiO₃—TiO₂ composite layer having an average thickness of2-12 μm adjacent the conductive transparent substrate, the crystallineCaTiO₃—TiO₂ composite layer comprising crystalline CaTiO₃—TiO₂ particleshaving an average diameter of 0.2-2.2 μm, wherein the crystallineCaTiO₃—TiO₂ composite layer comprises 25-87 wt % CaTiO₃ and 13-75 wt %TiO₂, each relative to a total weight of the crystalline CaTiO₃—TiO₂composite layer.
 9. The method of claim 8, wherein the calcium complexcomprises trifluoroacetate ligands, acetate ligands, isopropanolligands, and tetrahydrofuran ligands.
 10. The method of claim 9, whereinthe calcium complex has a formula[Ca₂(TFA)₃(OAc))(^(i)PrOH)(H₂O)(THF)₃].
 11. The method of claim 8,wherein before the contacting, the aerosol consists essentially of thecarrier gas, the calcium complex, the titanium complex, and the solvent.12. The method of claim 8, wherein the calcium complex and the solventare present in the aerosol at a weight ratio of 1:1000-1:5.
 13. Themethod of claim 8, wherein the titanium complex and the solvent arepresent in the aerosol at a weight ratio of 1:10,000-1:5.
 14. The methodof claim 8, wherein the aerosol is contacted with the conductivetransparent substrate for a time period of 10-120 min.
 15. The method ofclaim 8, wherein during the contacting, the carrier gas has a flow ratein a range of 20-250 mL/min. 16-20. (canceled)